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Frequently Asked Questions (FAQ) About Hot-Dip Galvanising for Fasteners

Section 1: The Basics and General Understanding

1. What exactly is hot-dip galvanising?
Hot-dip galvanising is an industrial metallurgical process where steel or iron components are completely immersed in a bath of molten zinc at approximately 450°C. This creates a thick, durable, metallurgically bonded coating that provides long-term corrosion protection—typically 50 to 100+ years in atmospheric environments.
2. Why is it called "hot-dip" galvanising?
The term "hot-dip" refers to the process of dipping (immersing) the steel component into hot molten zinc. This distinguishes it from other zinc coating methods like electroplating (which uses electricity at room temperature) or thermal spraying (which uses a spray gun).
3. What is the primary purpose of galvanising?
The primary purpose is to provide exceptional long-term corrosion protection for steel and iron components, especially those exposed to harsh outdoor, marine, industrial, or corrosive environments where rust would cause structural failure or significant maintenance costs.
4. Who invented galvanising and when?
The process was invented and patented by French engineer Stanislas Sorel in 1836. However, the electrochemical principle of sacrificial protection (on which galvanising relies) was discovered earlier by British scientist Sir Humphry Davy in 1824 when he used zinc plates to protect copper sheathing on naval ships.
5. How long does a galvanised coating last?
Service life depends on the environment's severity and the coating thickness. In rural or suburban atmospheric conditions, galvanised coatings routinely last 50-100+ years with no maintenance. In harsh industrial or marine coastal zones, expect 30-50 years. Even in severe marine splash zones, 20-30 years is typical. This far exceeds paint or plating, which may require recoating every 5-20 years.
6. What colour is a galvanised finish?
When fresh, galvanised steel is often bright and shiny with a metallic silver appearance, sometimes with a crystalline pattern called "spangle." As it weathers over the first 1-2 years, it develops a uniform, non-reflective matte grey colour. The appearance has no bearing on corrosion resistance.
7. Is hot-dip galvanising expensive?
The initial material and processing cost is higher than paint or thin electroplating. However, when lifecycle cost is considered—including maintenance, recoating, and replacement—galvanising is typically the most cost-effective solution. You pay once, and the coating lasts for decades with zero maintenance.
8. Can stainless steel be galvanised?
No. Galvanising stainless steel is neither standard practice nor recommended. Stainless steel is already highly corrosion-resistant due to its chromium content. The heat and molten zinc can cause liquid metal embrittlement and cracking in stainless steel. There is also no benefit—the galvanised coating would offer less protection than the base stainless steel.
9. What is "white rust" and is it a problem?
White rust (also called "wet storage stain") is a white, chalky, powdery deposit that can form on newly galvanised surfaces if they are stored in damp, poorly ventilated conditions with surfaces in tight contact (preventing air circulation). It is zinc hydroxide or zinc carbonate formed by reaction with moisture and carbon dioxide. It is primarily a cosmetic issue. If white rust is light and superficial, it usually does not significantly reduce the coating's long-term protective ability. If heavy and deep, it may reduce coating thickness. Prevention: Store galvanised components in a dry, well-ventilated area with spacers between items to allow air circulation.
10. Are galvanised components safe to handle?
Yes. Once cooled and solidified, galvanised coatings are completely safe to handle with bare hands. Zinc is an essential trace element in human nutrition. There are no toxic fumes or hazards from handling galvanised products. The only safety concern is during welding or cutting (see FAQ Section 5 for details on zinc fume hazards).

Section 2: The Process and Technical Details

11. What are the main stages of the galvanising process?
There are three primary stages:

1. Surface Preparation: Degreasing (removing oil/grease), pickling (removing rust and mill scale with acid), and fluxing (final cleaning and protection).
2. Galvanising: Full immersion in molten zinc at 450°C, forming zinc-iron alloy layers. For fasteners, this includes centrifuging to clear threads.
3. Inspection: Visual checks, coating thickness measurement, and thread gauging.

12. What is the difference between hot-dip galvanising and spin galvanising?
"Spin galvanising" is not a separate process—it is a specific part of the hot-dip galvanising process used for small components like nuts, bolts, and washers. After immersion in molten zinc, the basket of parts is immediately placed in a high-speed centrifuge and spun. This removes excess molten zinc from threads, holes, and small details, ensuring they remain functional and not clogged with zinc.
13. How thick is a hot-dip galvanised coating?
Coating thickness depends on the standard specified and the thickness of the base steel. Typical values:

• Fasteners (per ISO 10684): 45-100 microns (µm) average.
• Structural steel >6mm (per ISO 1461): 85 µm average minimum, 70 µm local minimum.
• Heavy structural components: Can exceed 100-150 µm.

For comparison, electroplated zinc is typically only 5-15 µm—up to 20 times thinner.
14. What is the temperature of the molten zinc bath?
Typically 445-455°C (833-851°F). Some galvanisers operate slightly lower (440°C) or higher (up to 465°C) depending on the steel type and coating requirements. The zinc remains molten at this temperature (zinc melts at 419.5°C).
15. How long do components stay in the zinc bath?
Immersion time varies with component mass and thickness. Small fasteners may only need 3-5 minutes. Heavy structural sections may require 8-15 minutes or more. The components must remain immersed until they reach thermal equilibrium with the zinc (i.e., their temperature equals the bath temperature). This is when the zinc-iron alloy reaction is complete.
16. What causes the crystalline "spangle" pattern on some galvanised surfaces?
Spangle is formed by the crystallisation of the pure zinc outer layer as it cools. The size and visibility of the spangle depend on the cooling rate, bath chemistry (especially lead content), and whether the surface is quenched or air-cooled. Large spangles form with slow cooling and higher lead content. Modern low-lead zinc and faster cooling produce smaller or no visible spangle. Spangle is purely cosmetic—it has no effect on corrosion resistance or coating performance.
17. Why do some galvanised surfaces look dull grey and others look shiny?
Freshly galvanised surfaces are often bright and shiny. As the zinc surface oxidises (reacts with oxygen and moisture in the air), it develops a thin layer of zinc oxide and zinc carbonate, which has a matte grey appearance. This patina is protective and stable. The rate of patina formation depends on environmental conditions—faster in humid or polluted areas, slower in dry rural conditions. Additionally, some steels (reactive steels with high silicon content) produce thicker, darker grey coatings immediately after galvanising.
18. Does the heat of galvanising weaken or change the steel?
For standard structural carbon steels and low-alloy steels (tensile strength up to 900 MPa, such as Grade 8.8 bolts), the 450°C galvanising temperature has no effect on mechanical properties. These steels are not heat-treated or tempered at such low temperatures. For very high-strength quenched and tempered steels (Grade 10.9, 12.9, or higher with tensile strength ≥1000 MPa), there can be a small reduction in hardness and strength due to tempering effects. Special procedures and post-galvanising testing may be required. Always consult the steel and fastener specifications.
19. What is oversize tapping, and why is it necessary?
Oversize tapping is the process of cutting the internal thread of a nut larger than the standard dimension after the nut has been hot-dip galvanised. It is absolutely essential because the thick zinc coating (50-100 µm) on a galvanised bolt increases the thread diameter. A standard nut will not fit onto a galvanised bolt—it will jam immediately. The oversize tapped nut has the zinc removed from its internal threads, creating clearance for the coated bolt thread while still maintaining sufficient thread engagement for full strength.
20. What happens if I try to use a standard nut on a galvanised bolt?
The nut will not fit. It will bind and jam on the first or second thread. If you attempt to force it (using excessive force or an impact wrench), the threads will gall (cold weld together) and seize completely. Both the bolt and nut will be destroyed and unusable. The assembly will fail. Never mix standard nuts with galvanised bolts. Always use correctly oversize tapped nuts.
21. Does oversize tapping weaken the nut?
No. The oversize dimensions specified in standards like ISO 10684 and ASTM F2329 are carefully calculated to ensure that even with the larger thread diameter, the nut retains sufficient thread engagement and cross-sectional area to meet or exceed the proof load and tensile strength requirements of the corresponding bolt grade. The nut remains fully load-rated.
22. What is hydrogen embrittlement, and should I be concerned?
Hydrogen embrittlement is a phenomenon where atomic hydrogen, absorbed by steel during acid pickling, diffuses into the steel lattice and reduces its ductility, making it brittle and prone to sudden cracking under stress. It is only a concern for very high-strength fasteners (typically Grade 10.9, 12.9, ASTM A490, or higher with tensile strength ≥1000 MPa). Standard structural grade fasteners (Grade 4.6, 5.8, 8.8) are not susceptible. Prevention: High-strength fasteners must be baked (heat-treated) at 190-220°C for several hours immediately after pickling to drive out absorbed hydrogen. This is specified in ASTM F2329 Supplementary Requirement S1 and ISO 10684 Annex C.
23. Can I specify the coating thickness I require?
Yes. Most standards (ISO 1461, ASTM A153) provide different thickness classes or grades. You can specify a thicker coating for more severe environments or longer design life. However, be aware that excessively thick coatings can become more brittle and may cause dimensional issues with threaded fasteners. Discuss requirements with your galvaniser.
24. What is "reactive steel" and why does it matter?
Reactive steel is steel with a silicon (Si) content typically between 0.15% and 0.25%. These steels react more rapidly with molten zinc, producing much thicker coatings (often 150-300 µm or more) that are darker grey, rougher, and more brittle than normal. This can be problematic for dimensional tolerance and appearance. If coating thickness control is critical, specify low-silicon steels (Si <0.04%) or request that the galvaniser use nickel or aluminium additions to the zinc bath to moderate the reaction.

Section 3: Application, Use, and Assembly

25. Can I weld galvanised steel?
Yes, but specific safety precautions and procedures are mandatory. Welding galvanised steel produces toxic zinc oxide fumes that can cause "metal fume fever" (flu-like symptoms). You must work in a very well-ventilated area or outdoors, use local fume extraction, and wear appropriate respiratory protection. Grind away the zinc coating 50-100mm either side of the weld zone before welding. After welding, the bare steel must be protected with zinc-rich paint, cold zinc spray, or other corrosion protection.
26. Can I paint over a galvanised surface?
Yes. This is called a "duplex system" and provides exceptional corrosion protection—far better than galvanising or paint alone. However, proper surface preparation is essential. The galvanised surface must be weathered (left outdoors for 6-12 months), chemically treated with T-Wash or etch primer, or lightly abraded to ensure good paint adhesion. Use paint systems specifically formulated for galvanised substrates. Consult paint manufacturer's technical data sheets.
27. Are galvanised fasteners suitable for marine environments?
Yes. Hot-dip galvanised fasteners perform very well in marine atmospheric zones (coastal areas, docks, offshore platforms above the splash zone). For direct immersion in seawater, consider more specialised materials (stainless steel, super duplex stainless, titanium, or heavy hot-dip galvanised with additional coating). For fasteners in tidal or splash zones, hot-dip galvanising to ASTM F2329 or ISO 10684 is often specified and performs well, though with a shorter service life (20-40 years) than in atmospheric conditions.
28. Can I use galvanised fasteners in contact with treated timber (CCA, ACQ, tanalised wood)?
Yes, but care is required. Copper-based timber preservatives (CCA - Copper Chrome Arsenic, or ACQ - Alkaline Copper Quaternary) are corrosive to zinc. Hot-dip galvanised fasteners are generally suitable for use with treated timber, as the thick coating provides adequate protection for the expected service life. For critical structural connections or very long design life, consider stainless steel fasteners (Grade 304 or 316). Always follow the timber treatment manufacturer's recommendations.
29. Can I mix galvanised fasteners with non-galvanised (plain steel) fasteners in the same assembly?
This is not recommended. Using a galvanised bolt with a plain steel nut (or vice versa) creates a galvanic cell when moisture is present. The zinc coating will corrode rapidly to protect the plain steel, significantly reducing the service life of the galvanised component. For best results, use a matched galvanised assembly: galvanised bolt + oversize tapped galvanised nut + galvanised washers.
30. Can I mix galvanised fasteners with stainless steel fasteners or components?
This should generally be avoided in corrosive environments. Stainless steel is much more noble (cathodic) than zinc on the galvanic series. When in electrical contact with moisture present, the zinc on the galvanised fastener will corrode very rapidly to protect the stainless steel, drastically shortening the galvanised component's life. If this combination cannot be avoided, electrically isolate the two metals using nylon or plastic washers and bushings, or apply a barrier coating (paint or tape) at the interface.
31. Can I use galvanised fasteners with aluminium components?
Generally, yes. Zinc and aluminium are relatively close on the galvanic series, so the galvanic corrosion rate is slow and often acceptable for many applications. This combination is commonly used in construction, automotive, and marine industries. However, in very corrosive environments (e.g., marine splash zones), some accelerated corrosion may occur. For critical applications, consider using aluminium fasteners, stainless steel fasteners, or applying an insulating barrier.
32. Are galvanised fasteners magnetic?
Yes. The base material is carbon steel or low-alloy steel, which is ferromagnetic. The zinc coating itself is non-magnetic, but the overall fastener is magnetic due to the steel substrate.
33. Should I lubricate galvanised threads before assembly?
It is not always necessary, but it is often beneficial, especially for larger diameter fasteners (M20 and above). Lubricating the threads can:

• Reduce friction and provide more consistent torque-tension relationship.
• Prevent galling and seizure during assembly, especially in thick or heavily coated threads.
• Allow you to achieve the specified bolt pre-load (tension) more reliably.

Use a suitable lubricant compatible with galvanised surfaces—typically a light oil, anti-seize compound, or wax-based thread lubricant. Avoid thick greases that can trap moisture.
34. How do I tighten galvanised bolts correctly? Are torque values different?
The coefficient of friction for galvanised threads is different from plain or plated steel. Galvanised surfaces are typically rougher and have higher friction. This means that for a given torque value, you will achieve less bolt tension than with a plain or lubricated bolt. Published torque values for standard bolts may not be accurate for galvanised assemblies. Best practice:

• Use torque values specifically developed for galvanised fasteners (consult engineering specifications or fastener supplier).
• For critical structural or high-strength bolted connections, use direct tension indicating methods: load cells, bolt tension calibrators, or direct tension indicators (DTIs).
• If torque wrenches are used, conduct calibration tests to establish the correct torque-tension relationship for your specific fastener assembly.

35. Can galvanised nuts be re-used after disassembly?
Yes, in most cases, as long as the threads are not damaged. Unlike thread-locking fasteners (e.g., nylon insert lock nuts, deformed thread lock nuts), standard galvanised hexagon nuts do not lose their locking ability upon removal. However, inspect threads carefully for galling, damage, or excessive wear before re-use. For critical or safety applications, always use new fasteners.
36. Can you hot-dip galvanise fasteners with a nylon locking insert (e.g., Nyloc nuts)?
No. The nylon insert would be completely destroyed by the 450°C temperature of the zinc bath. The correct procedure is:

1. Galvanise the nut body (without the nylon insert) first.
2. After galvanising, allow the nut to cool.
3. The nylon locking insert is then pressed or machined into the nut.

The finished product is a galvanised nut with a nylon locking feature. These are available as standard products from fastener suppliers.
37. What is the difference between "galvanised finish" and "sherardised finish"?
Both are zinc coating processes, but fundamentally different:

• Hot-Dip Galvanising: Immersion in molten zinc at 450°C. Produces a thick (50-100+ µm) coating.
• Sherardising: A lower-temperature diffusion process (350-400°C) where components are tumbled in a drum with zinc dust in the absence of air. Zinc diffuses into the steel surface, forming a thin (10-40 µm) zinc-iron alloy coating.

Sherardising is often used for small, precision fasteners and threaded components where dimensional tolerance is critical, as it produces a thinner, more uniform coating that does not significantly affect thread fit. However, it offers less corrosion protection than hot-dip galvanising.
38. Can I use galvanised fasteners in high-temperature applications?
Galvanised fasteners are suitable for continuous service temperatures up to approximately 200°C. Above this temperature:

• The zinc coating begins to oxidise more rapidly.
• At ~300-400°C, the zinc coating may start to diffuse further into the steel or evaporate.
• Above 419°C (the melting point of zinc), the coating would melt and run off.

For high-temperature applications (>200°C), consider alternative materials: stainless steel fasteners (service to 800°C+), high-temperature alloys (Inconel, Hastelloy), or ceramic coatings.
39. Are there any chemicals that will rapidly attack galvanised coatings?
Yes. Zinc coatings are vulnerable to strong acids and strong alkalis:

• Strong Acids (pH <4): Hydrochloric acid, sulphuric acid, nitric acid, and organic acids will rapidly dissolve zinc. Even weak acidic environments (e.g., acid rain, acidic soils, acidic drainage from concrete or timber) will accelerate zinc corrosion.
• Strong Alkalis (pH >12.5): Sodium hydroxide, potassium hydroxide, lime (calcium hydroxide), and fresh concrete or mortar (highly alkaline) will attack zinc.
• Salts: Chlorides (from seawater, de-icing salts, or industrial pollution) accelerate zinc corrosion, though galvanised coatings still provide good protection—just with reduced service life.

If exposure to aggressive chemicals is expected, consider alternative coatings (epoxy, polymer, or stainless steel).
40. Can galvanised fasteners be used underground or in soil?
Yes, but with reduced service life compared to atmospheric exposure. In most soils (neutral pH, well-drained), galvanised coatings provide 20-50 years of protection. In aggressive soils (acidic, high chloride content, waterlogged, or contaminated), service life may be significantly reduced (10-20 years). For long-term underground applications, consider increasing coating thickness, using a duplex system (galvanising + organic coating), or specifying corrosion-resistant alloys.

Section 4: Standards, Specification, and Quality Control

41. What is the main difference between ISO 1461 and ASTM A153?
Both are primary standards for hot-dip galvanised coatings, but they have regional origins and some technical differences:

• ISO 1461: The dominant international and European standard. Used in UK, EU, Australia, and many other countries. Specifies coating thickness based on steel thickness with clear minimum local and average requirements.
• ASTM A153: The primary US standard for galvanised hardware and fasteners. Specifies coating weight (g/m²) rather than thickness (µm), though weight and thickness are directly related. Uses three classes (A, B, C) based on component thickness.

For international projects, ISO 1461 or ISO 10684 (for fasteners) is often preferred for harmonisation. For US projects, ASTM A153 or ASTM F2329 is standard.
42. What is ISO 10684, and why is it important for fasteners?
ISO 10684 is the international standard specifically written for hot-dip galvanised fasteners (bolts, screws, nuts, washers). It addresses the unique requirements of threaded components, including:

• Coating thickness requirements for different fastener sizes.
• Centrifuging procedures to clear threads of excess zinc.
• Detailed specifications for oversize tapping of nuts (Class G dimensions) to ensure fit with galvanised bolts.
• Thread gauging and functional testing requirements.

This standard is essential when specifying galvanised fasteners for structural or critical applications.
43. If a drawing calls for the old BS 729 standard, what should I use now?
BS 729:1971 was the former British Standard for hot-dip galvanising. It was withdrawn and replaced by BS EN ISO 1461. If a legacy drawing or specification references BS 729, the modern equivalent is:

• For structural fabrications: BS EN ISO 1461:2022
• For fasteners: BS EN ISO 10684:2004+A1:2012

The requirements are very similar, though ISO 1461 has more detailed thickness classifications. Always confirm with the client or specifying engineer that updating to the current standard is acceptable.
44. How do I specify an oversize tapped nut correctly on a drawing or purchase order?
The specification should include:

• Nut size, thread type, and pitch (e.g., M20 x 2.5, or 3/4"-10 UNC).
• Material grade (e.g., Grade 8, ASTM A563 Grade DH).
• Coating standard: "Hot-dip galvanised to BS EN ISO 10684" or "Hot-dip galvanised to ASTM F2329".
• Critical requirement: "Tapped oversize Class G (or Class 2G) after galvanising for assembly with hot-dip galvanised bolts."

This makes it absolutely clear that the nut must be retapped after galvanising.
45. Does galvanising affect the grade marking or identification on a bolt head?
Sometimes. The zinc coating can obscure or partially fill in grade markings, manufacturer's marks, or other identification stamped or rolled into the bolt head. ISO 10684 and ASTM F2329 include provisions for ensuring markings remain legible. Options include:

• Deeper or larger markings applied before galvanising.
• Re-striking (re-stamping) markings after galvanising (this removes zinc from the marking area).
• Alternative identification methods (colour coding, tagging, or documentation).

For critical applications, specify that markings must remain legible after galvanising.
46. What is a "Preece test"?
The Preece Test (also called the Copper Sulphate Test) is a simple, standardised quality control test used to assess the uniformity and freedom from defects of a galvanised coating. A sample component is repeatedly dipped in a copper sulphate solution for 1-minute intervals. After each immersion, it is rinsed and inspected. When copper (pink/red colour) deposits on the surface, this indicates the zinc coating has been penetrated and bare steel is exposed. The number of 1-minute dips before copper deposition occurs is the Preece Test result. ISO 1461 specifies minimum requirements (e.g., 4 dips for steel >6mm thick). This test is destructive and performed on a sample basis, not on every component.
47. How is coating thickness measured on-site or during inspection?
Non-destructive magnetic (electromagnetic induction) coating thickness gauges are used. These are handheld digital instruments with a probe that is placed on the galvanised surface. The gauge measures the distance from the probe to the steel substrate (i.e., the coating thickness) using magnetic principles. The gauge must be calibrated using certified thickness standards before use. Common gauge models: Elcometer, DeFelsko PosiTector, Fischer, Quanix. Gauges must conform to BS EN ISO 2178 or ASTM E376.
48. What is a galvanising certificate, and what should it include?
A galvanising certificate (also called a coating certificate or process certificate) is a document provided by the galvaniser confirming that the work has been completed to the specified standard. It should include:

• Galvaniser's company name, address, and contact details.
• Customer name and order/batch reference number.
• Description and quantity of components galvanised.
• Material specification (if known).
• Standard to which galvanising was performed (e.g., BS EN ISO 1461:2022).
• Coating thickness measurements: minimum, maximum, and average values (in microns).
• Confirmation that visual inspection and adhesion testing have been satisfactorily completed.
• Date of galvanising.
• Authorised signature of galvanising plant manager or quality inspector.
• Galvaniser's accreditation details (e.g., ISO 9001, CARES, HDGASA membership).

49. What is CARES approval, and does it matter?
CARES (Certification Authority for Reinforcing Steels) is a UK-based independent product certification scheme. While originally focused on reinforcing steel, CARES also operates the CARES Steelwork Certification Scheme, which includes assessment and approval of galvanising plants. A CARES-approved galvaniser has demonstrated compliance with relevant standards (ISO 9001 quality management, ISO 1461 technical compliance) and undergoes regular third-party audits. Specifying a CARES-approved galvaniser provides additional assurance of quality and consistency. For critical structural or public infrastructure projects in the UK, CARES approval is often a contract requirement.
50. Can I request third-party inspection or testing of galvanised fasteners?
Yes. For critical applications, high-value projects, or where additional assurance is required, you can engage an independent third-party inspection agency (e.g., Bureau Veritas, SGS, Lloyds Register, TÜV) to witness the galvanising process, conduct inspections, perform testing, and issue independent certification. Costs are additional, but this is standard practice for offshore, nuclear, defence, and large infrastructure projects.

Section 5: Troubleshooting, Defects, and Remediation

51. What are the most common defects in galvanised coatings, and what causes them?

• Bare Spots or "Skip": Areas where zinc failed to bond. Caused by inadequate surface preparation (residual oil, scale, or flux not removed). Prevention: Rigorous degreasing and pickling.
• Dross Inclusions: Rough, grey lumps of zinc-iron dross embedded in the coating. Caused by poor bath maintenance or stirring up dross from the bath bottom. Prevention: Regular bath skimming and good process control.
• Flux Inclusions: Black or dark grey deposits. Caused by inadequate rinsing after fluxing or flux becoming trapped in corners or joints. Prevention: Thorough rinsing, good drainage, and proper component design.
• Runs or Sags: Excessive zinc build-up at bottom edges. Caused by incorrect withdrawal speed or draining angle. Prevention: Controlled withdrawal and proper jigging.
• Rough or Spikey Surface ("Tow Hair"): Very rough texture with whiskers of zinc. Caused by reactive steel (high silicon) or incorrect bath chemistry. Prevention: Use low-silicon steel or aluminium/nickel bath additions.
• Peeling or Flaking: Coating detaches in sheets. Caused by very poor adhesion due to surface contamination. Prevention: Proper surface preparation. Remedy: Strip and re-galvanise.

52. If I discover bare spots or thin coating after galvanising, what can I do?
For small areas:

• Zinc-Rich Paint: Apply a two-part zinc-rich epoxy primer (95% zinc dust). This provides cathodic protection similar to galvanising.
• Cold Zinc Spray: Apply a cold zinc spray coating (aerosol or spray gun). Less effective than zinc-rich paint, but acceptable for small repairs.
• Zinc Sticks or Thermal Spray: For larger areas, thermal arc or flame spray can be used to apply molten zinc to bare areas.

For large areas or critical structural components: The component should be stripped (de-galvanised in acid) and re-galvanised. This is the only way to ensure full compliance with specifications.
53. How do you remove or strip a galvanised coating?
Galvanised coatings can be chemically stripped (de-galvanised) by immersing the component in a dilute acid solution (typically hydrochloric acid at 5-10% concentration or sulphuric acid). The acid dissolves the zinc coating, leaving the steel substrate intact (though slightly etched). After stripping, the component is rinsed, re-prepared (pickled and fluxed), and re-galvanised. This is done when coating defects are unacceptable or when components need to be re-worked.
54. Can galvanised components be machined or drilled after galvanising?
Yes, but any machined or drilled surfaces will expose bare steel. These areas must be protected. Options:

• Apply zinc-rich paint or cold zinc spray to the newly exposed surfaces immediately after machining.
• For high-quality or critical applications, have the components re-galvanised after machining (though this is expensive and often impractical).

Best practice: Design components so that all machining, drilling, and tapping operations are completed before galvanising.
55. What should I do if threads are clogged or bridged with zinc after galvanising?
This indicates that centrifuging was inadequate, not performed, or incorrect. Options:

• Re-tapping (Internal Threads): Run a tap through the thread to cut away excess zinc and restore the thread profile. This is standard practice for nuts that need to be tapped oversize anyway.
• Die Chasing (External Threads): Run a thread-cutting die over external threads to remove excess zinc. This is less ideal, as it removes the protective coating from the thread flanks. May be acceptable for non-critical applications.
• Mechanical Cleaning: Use a wire brush, thread file, or thread-chasing tool to remove excess zinc. Labour-intensive and often incomplete.
• Return to Galvaniser: If centrifuging was clearly inadequate, the batch may be rejected and returned to the galvaniser for re-processing.

56. Can I repair damaged galvanised coatings on-site?
Yes, for minor damage (scratches, small bare areas, handling damage). Use zinc-rich paint or cold zinc spray as a field repair. For best results:

• Clean the damaged area (remove dirt, oil, loose zinc).
• Lightly abrade the surrounding galvanised surface (to promote adhesion).
• Apply zinc-rich paint or spray according to manufacturer's instructions.
• Typical products: Galvafroid, Zinga, Bison Zinc, or equivalent (available from industrial coatings suppliers).

For severe damage or large areas, consult a corrosion engineer or consider replacing the component.
57. What causes galvanised coatings to crack or spall (flake off in chunks)?
Cracking or spalling typically indicates:

• Excessively Thick Coating on Reactive Steel: Very thick alloy layers (>200 µm) can be brittle and crack under impact or bending. Use low-silicon steel or bath chemistry control.
• Thermal Shock: Rapid cooling (especially cold water quenching) of heavy, thick components can cause coating stress and cracking. Use slower cooling methods.
• Bending or Forming After Galvanising: Galvanised coatings have limited ductility. Bending galvanised components can cause the coating to crack. Best practice: Form and bend before galvanising. If post-galvanising bending is unavoidable, use larger bend radii and slower bending speeds.
• Liquid Metal Embrittlement: Rare, but can occur with high-strength steels under stress. Causes sudden cracking during galvanising.

58. Why has my galvanised coating started to "sweat" or "bleed" after installation?
This is usually "wet storage stain" (white rust) or delayed flux bleeding. It can occur if:

• Flux residues were trapped in joints, laps, or blind holes and are now leaching out with moisture.
• The component was stored or installed in very humid or wet conditions immediately after galvanising, before the zinc oxide patina fully formed.

Prevention: Ensure proper drainage holes in hollow fabrications, avoid tight stacking without spacers, store in dry ventilated conditions for at least 48 hours after galvanising before exposure to wet conditions.

Section 6: Comparison with Other Coatings and Alternatives

59. How does hot-dip galvanising compare with powder coating for corrosion protection?

• Hot-Dip Galvanising: Provides cathodic (sacrificial) protection. Even if scratched to the substrate, zinc protects the exposed steel. Service life 50-100+ years in atmospheric conditions. Very high abrasion resistance. Dull grey appearance.
• Powder Coating: Provides only barrier protection. If scratched to the substrate, corrosion begins immediately at that location (no self-healing). Service life typically 10-25 years depending on coating thickness and environment. Easily chipped or scratched. Excellent aesthetic finish—available in any colour, gloss, or texture.
• Duplex System (Galvanising + Powder Coating): Combines the best of both—cathodic protection from galvanising + barrier protection and aesthetics from powder coating. Provides 1.5-2.5 times the service life of either system alone. This is the premium solution for maximum protection and appearance.

60. How does hot-dip galvanising compare with stainless steel for corrosion resistance?
Both are excellent, but suited to different applications:

• Hot-Dip Galvanising: Applied to standard carbon steel. Lower material cost. Very thick protective coating. Service life 50-100+ years in most atmospheric environments. Can be damaged by strong acids or alkalis. Suitable up to ~200°C continuous service.
• Stainless Steel (Grade 304/316): Inherently corrosion-resistant (no coating required). Higher material cost. Excellent resistance to most chemicals and environments. Susceptible to chloride-induced pitting and crevice corrosion in some conditions (316 is better than 304). Suitable for high-temperature service (up to 800°C+).
• Cost vs. Performance: For large structural applications where cost is critical and environment is atmospheric, galvanised carbon steel is more economical. For smaller fasteners in aggressive chemical or marine immersion environments, stainless steel may be more cost-effective over the lifecycle.

61. What is "mechanical galvanising" and how does it differ from hot-dip galvanising?
Mechanical Galvanising (also called peen plating or impact plating) is a cold process where components are tumbled in a drum with zinc powder, glass beads, and proprietary chemicals. The mechanical impact bonds zinc to the steel surface. Differences:

• No Heat: Room temperature process—no risk of distortion or hydrogen embrittlement. Suitable for high-strength fasteners where hot-dip is problematic.
• Thinner Coating: Typically 20-50 µm (thinner than hot-dip, but thicker than electroplating).
• Coating Structure: Pure zinc layers, not zinc-iron alloy layers. No metallurgical bond.
• Applications: Often used for small fasteners, especially high-strength or heat-treated components.

Mechanical galvanising is less common than hot-dip galvanising or electroplating, and is typically more expensive for large volumes.
62. What is "zinc flake coating" (Geomet, Dacromet, Magni) and when is it used instead of galvanising?
Zinc flake coatings are ultra-thin (<10 µm) coatings composed of zinc and aluminium flakes in an inorganic binder. They are applied by dipping or spraying and then cured in an oven at ~300°C. Advantages:

• No hydrogen embrittlement risk: Suitable for high-strength fasteners.
• Excellent corrosion resistance: 500-1000+ hours salt spray resistance (comparable to thicker coatings).
• Minimal thickness: No effect on thread fit—standard nuts and bolts can be used together.
• Disadvantages: Higher cost. Less abrasion resistance. Not suitable for long-term outdoor structural use (primarily automotive, aerospace, and industrial fasteners).

Common brand names: Geomet, Dacromet, Magni, Delta-Protekt, Doerken MKS.

Section 7: Environmental, Health, and Safety

63. Is zinc toxic? Are there environmental concerns with galvanising?
Zinc is an essential trace element for human, animal, and plant health. It is naturally present in soil, water, and the environment. Zinc is not classified as toxic at environmental concentrations. However:

• Aquatic Toxicity: High concentrations of dissolved zinc can be toxic to aquatic life (fish, algae). Galvanising plants must treat wastewater to remove dissolved zinc before discharge (typically using precipitation and filtration). This is standard environmental practice and regulated by environmental agencies.
• Zinc Fumes: Inhaling zinc oxide fumes (from welding or cutting galvanised steel) can cause "metal fume fever"—temporary flu-like symptoms. This is not a chronic or long-term health issue, but it is unpleasant. Proper ventilation and respiratory protection are essential.
• Sustainability: Zinc is 100% recyclable. At the end of a structure's life, galvanised steel is recycled in electric arc furnaces. The zinc is recovered and reused. Modern galvanising plants operate closed-loop acid recycling systems and zinc dross recovery, minimising waste.

64. What is "metal fume fever" (zinc shakes) and how is it prevented?
Metal fume fever (also called "zinc shakes" or "galvaniser's flu") is a temporary illness caused by inhaling freshly generated zinc oxide fumes. Symptoms:

• Flu-like symptoms: fever, chills, muscle aches, headache, nausea, fatigue.
• Onset: 4-12 hours after exposure.
• Duration: 12-48 hours. Full recovery with no long-term effects.
• Cause: Breathing zinc oxide fumes from welding, cutting, or torch cutting galvanised steel.

Prevention:

• Work in a well-ventilated area or outdoors.
• Use local exhaust ventilation (LEV) or fume extraction at the source.
• Wear appropriate respiratory protection (P2 or P3 rated particulate filter, or supplied air respirator if ventilation is inadequate).
• Grind away zinc coating from weld zones to reduce fume generation.
• Inform all workers of the hazard before work begins.

Treatment: Rest, fluids, and over-the-counter pain relief. Symptoms resolve without medical treatment in most cases.
65. Are there any regulations or standards for welding galvanised steel safely?
Yes. In the UK, the Control of Substances Hazardous to Health (COSHH) Regulations 2002 require employers to assess risks from hazardous substances (including zinc oxide fumes) and implement controls. The HSE (Health and Safety Executive) provides guidance document EH40/2005 "Workplace Exposure Limits," which specifies the maximum allowable zinc oxide fume concentration in workplace air: 10 mg/m³ time-weighted average (TWA) and 50 mg/m³ short-term exposure limit (STEL). Employers must ensure exposure does not exceed these limits through ventilation, extraction, and respiratory protection. Similar regulations exist in other countries (OSHA in the USA, Safe Work Australia, etc.).
66. Can galvanised fasteners be recycled?
Yes, absolutely. Galvanised steel is 100% recyclable. At the end of service life, galvanised steel is collected and melted in electric arc furnaces (EAF) or basic oxygen furnaces (BOF) for steelmaking. The zinc either:

• Evaporates as zinc vapour (due to zinc's low boiling point ~900°C vs. steel's melting point ~1500°C) and is captured in fume recovery systems, where it is condensed and recovered as zinc oxide for reuse in zinc smelting.
• Mixes into the molten steel in small amounts, or floats to the surface as dross and is skimmed off.

Recycled steel retains its properties and can be used indefinitely. Galvanised steel scrap is valuable and widely accepted by scrap metal recyclers.

Section 8: Advanced Topics and Special Applications

67. What is "double galvanising" or "re-galvanising"?
This refers to galvanising a component twice. It is occasionally done when:

• The first galvanising pass produced an inadequate or thin coating (e.g., on reactive steel).
• A component has been machined or modified after galvanising and needs additional protection.
• Extreme corrosion resistance is required (e.g., offshore or subsea applications).

The component is galvanised once, cooled, and then re-immersed in the zinc bath. The result is a very thick coating (150-250+ µm). However, this can make the coating brittle and cause dimensional issues, so it is rarely done for fasteners. More commonly used for large structural components or pipework.
68. Can you selectively galvanise only part of a component?
Yes, this is called "partial galvanising" or "stop-off galvanising." Areas that must not be galvanised are masked using special high-temperature resistant stop-off paints, tapes, or mechanical plugs before immersion in the zinc bath. After galvanising, the masking is removed, leaving those areas uncoated. Applications include:

• Machined surfaces that must remain precise (bearing surfaces, datum faces).
• Threaded areas that must remain standard size (though oversize tapping is usually a better solution).
• Contact surfaces for electrical conductivity.

However, stop-off galvanising is labour-intensive, expensive, and the masked areas will have no corrosion protection. Use only when absolutely necessary.
69. What is "centrifugal galvanising" and when is it used?
Centrifugal galvanising is a specialist process for hollow tubular components (pipes, hollow sections, tubes). The component is rotated at high speed while molten zinc is poured or sprayed into the bore. Centrifugal force distributes the zinc evenly across the internal surface, coating the inside of the tube. This is used for:

• Water pipes and pressure vessels where internal corrosion protection is critical.
• Hollow structural sections that cannot be vented for full immersion galvanising.

It is not commonly used for fasteners, but is important in pipeline and pressure vessel applications.
70. Can you galvanise cast iron?
Yes, but with limitations. Grey cast iron can be galvanised, though the coating thickness may be uneven due to the material's composition (high carbon and silicon content). The coating may also be more brittle. Ductile (nodular) cast iron galvanises more successfully, producing coatings similar to steel. Malleable cast iron also galvanises well. Applications include:

• Cast iron pipe fittings, valves, and drainage components.
• Architectural and street furniture castings.

Consult with the galvaniser before specifying galvanising of cast iron components.
71. What is "dry galvanising" or "pre-fluxing"?
This is an alternative surface preparation method where the flux solution is applied and then dried onto the component surface before it enters the zinc bath (wet fluxing). The dried flux layer protects the steel from oxidation during transport to the kettle. When the component enters the molten zinc, the flux melts and performs its function. Pre-fluxing is common in continuous galvanising lines and some batch galvanising operations. The alternative is "top fluxing," where a molten flux blanket floats on top of the zinc bath, and components pass through this layer immediately before immersion.
72. What is "long-term atmospheric corrosion testing" and how is it used to predict service life?
Long-term atmospheric corrosion testing involves exposing galvanised test panels to real-world environments (rural, urban, industrial, marine) for extended periods (10-30+ years) and measuring zinc loss over time. This data is used to calculate zinc corrosion rates in different environments and predict coating service life. Organisations like the International Zinc Association (IZA), British Steel, and CSIRO have conducted extensive testing. Results show:

• Rural environments: 0.5-1.5 µm zinc loss per year.
• Urban/Suburban: 1.5-3 µm per year.
• Industrial: 3-6 µm per year.
• Marine coastal: 4-8 µm per year.
• Marine splash zone: 8-15+ µm per year.

Using these rates, service life can be estimated: Service Life (years) ≈ Coating Thickness (µm) ÷ Corrosion Rate (µm/year). Example: 85 µm coating in urban environment (2.5 µm/year) = ~34 years to first rust. However, zinc corrosion slows over time as protective corrosion products build up, so actual service life is often 1.5-2 times the calculated value.

Section 9: Fastener-Specific Questions

73. What sizes of galvanised fasteners does Trojan Special Fasteners supply?
Trojan Special Fasteners can supply hot-dip galvanised fasteners in an extensive size range:

• Metric: M3, M4, M5, M6, M7, M8, M9, M10, M11, M12, M13, M14, M15, M16, M17, M18, M20, M22, M24, M25, M26, M27, M28, M30, M32, M33, M34, M35, M36, M38, M39, M40, M42, M45, M48, M50, M52, and M56 in coarse, fine, extra fine, and hard-to-find pitches.
• Imperial (BSW, BSF, BSP): 2BA to 2" in various threads per inch (TPI).
• Unified (UNF, UNC, UNEF): #8 to 2.1/4" in standard and special pitches.
• Available in both right-hand and left-hand threads. Oversize diameter options available for special applications.

74. Can Trojan supply galvanised fasteners with traceability and certification?
Yes. All galvanised fasteners supplied by Trojan Special Fasteners can be provided with:

• Full material traceability documentation.
• Material test certificates to EN 10204 3.1 or 3.2 (where required for critical applications).
• Hot-dip galvanising certificates from approved galvanisers, documenting coating thickness, process compliance, and conformity to specified standards (ISO 1461, ISO 10684, ASTM A153, ASTM F2329).
• Independent third-party inspection reports (if specified).

This ensures complete traceability from raw material through manufacturing, heat treatment, galvanising, and final inspection—essential for structural, offshore, nuclear, defence, and regulated industries.
75. What is the lead time for oversize tapping of galvanised nuts?
Lead times vary depending on quantity, size, and current workload. Typical lead times:

• Small quantities (up to 100 nuts): 3-5 working days.
• Medium quantities (100-1000 nuts): 1-2 weeks.
• Large quantities (1000+ nuts): 2-4 weeks.
• Urgent requirements: Express service available—contact our sales team.

For project planning, it's best to allow 2-3 weeks from order placement to delivery for custom oversize tapping work. Stock galvanised nuts in standard sizes are often available for immediate dispatch.
76. Can Trojan supply galvanised bolts with threads rolled after galvanising?
No, this is not standard practice. Thread rolling requires extremely high forces (50-200 tonnes) that would damage the galvanised coating. Standard practice is:

• Threads are cut or rolled before galvanising.
• The component is then galvanised.
• The thick coating on external threads is accommodated by using oversize tapped nuts.

If dimensional tolerance is absolutely critical and standard galvanising + oversize nuts is not acceptable, consider alternative coatings (zinc flake, mechanical galvanising, or electroplating) that produce thinner coatings with minimal dimensional change.
77. Are galvanised self-tapping screws and self-drilling screws available?
Yes, but with limitations. Self-tapping screws (with thread-forming or thread-cutting points) can be galvanised, though the coating on the forming/cutting edges may be thick enough to affect initial thread engagement. Self-drilling screws (with drill point) are more problematic—the thick zinc coating can clog the drill point flutes and reduce drilling efficiency. For high-volume self-tapping or self-drilling applications, zinc plating or zinc flake coatings are more commonly used. For lower-volume or non-critical applications, hot-dip galvanised self-tappers are available. Consult Trojan's technical team for specific requirements.
78. Can you galvanise stainless steel fasteners for additional protection?
No. This is neither recommended nor standard practice. Stainless steel is already highly corrosion-resistant. The molten zinc can cause liquid metal embrittlement of stainless steel, leading to cracking. There is also no benefit—the zinc coating would provide less corrosion resistance than the base stainless steel. If you require corrosion protection beyond standard stainless steel, consider:

• Higher-grade stainless steel (316, 316L, duplex, super duplex).
• Specialist coatings designed for stainless steel.
• Alternative alloys (titanium, Inconel, Hastelloy for extreme environments).

79. What is the typical roughness of a galvanised surface, and does it affect gasket sealing?
Galvanised surfaces are typically rougher than machined or plated steel. Surface roughness (Ra) is typically 5-15 µm for standard galvanising, though this can be higher on reactive steels or very thick coatings. For applications requiring gasket sealing or fluid-tight joints:

• Use soft, compressible gaskets (rubber, EPDM, neoprene, fibre) that can conform to the rough surface.
• Increase bolt tension to compress the gasket adequately.
• For critical high-pressure or vacuum applications, consider machining the sealing face after galvanising and protecting the machined surface with zinc-rich paint, or use a duplex system (galvanising + paint/coating).

80. Can I order galvanised fasteners to a specific coating thickness—for example, 100 µm minimum?
Yes, though this may require special processing. Standard galvanising to ISO 10684 produces 45-85 µm average coating on fasteners. If you require a thicker coating (e.g., 100 µm+ for extreme environments or extended service life):

• Specify "Heavy Galvanising" or "Class A coating per ASTM A153" (approximately 85 µm minimum).
• Discuss requirements with Trojan's technical team and the galvaniser. Thicker coatings may require longer immersion time, higher bath temperature, or use of reactive steel (which has drawbacks).
• Be aware that very thick coatings may be more brittle, have rougher surfaces, and cause greater dimensional changes (requiring larger oversize tapping of nuts).

For offshore, subsea, or extreme marine environments requiring maximum protection, coatings of 100-150 µm or duplex systems (galvanising + organic coating) can be specified.

Section 10: Cost, Specification, and Procurement

81. How much does hot-dip galvanising cost?
Galvanising cost depends on many factors: component size and weight, quantity, coating thickness required, complexity (venting, fixturing), and the galvaniser's location and capacity. Rough cost guidance (UK, 2024-2025):

• Small fasteners (per kg): £1.50-£3.00/kg.
• Medium structural components (per tonne): £200-£400/tonne.
• Large fabrications: £150-£300/tonne (economies of scale).
• Oversize tapping service: £0.50-£2.00 per nut (depending on size and quantity).

For accurate pricing, request a quotation from Trojan Special Fasteners or your chosen galvaniser with full component details, drawings, and quantities.
82. Is hot-dip galvanising more expensive than paint?
Initial cost: Yes, galvanising is typically 1.5-3 times more expensive than a standard industrial paint system. Lifecycle cost: No. When you factor in paint maintenance (surface preparation, recoating every 10-20 years, access costs, downtime), galvanising is usually far more economical over 30-50+ years. Galvanising is a "fit and forget" solution with zero maintenance. For long-life structures (bridges, transmission towers, offshore platforms, building structures), galvanising delivers the lowest total cost of ownership.
83. How do I specify galvanised fasteners on a technical drawing or bill of materials?
A complete specification should include:

• Size and thread: M20 x 2.5, 3/4"-10 UNC, etc.
• Length: For bolts and screws.
• Material grade: EN 8.8, ASTM A325, BS 3692 Grade 8, etc.
• Head style and drive: Hexagon head, socket head, etc.
• Coating standard: "Hot-dip galvanised to BS EN ISO 10684" or "Hot-dip galvanised to ASTM F2329."
• Nut specification (if applicable): "Hexagon nut, Grade 8, hot-dip galvanised, tapped oversize Class G per ISO 10684 for assembly with galvanised bolts."
• Washer specification: "Galvanised plain washer to BS 4320 / ISO 7089."
• Additional requirements: Traceability, certification, third-party inspection, etc.

84. Can I use galvanised fasteners as a direct substitute for plain or plated fasteners in an existing design?
Usually yes, but with important considerations:

• Thread Fit: If using galvanised bolts, you must use oversize tapped galvanised nuts—standard nuts will not fit.
• Dimensional Changes: The coating adds 0.1-0.2mm to all dimensions. Verify this does not cause clearance or fit issues (e.g., bolts passing through close-tolerance holes).
• Torque Values: Galvanised fasteners have different friction characteristics. Torque-tension relationship may change. Review and adjust tightening specifications.
• Thread Engagement: Ensure sufficient thread engagement remains after accounting for coating thickness.
• Appearance: Galvanising has a different appearance (matte grey) than plating (bright, shiny). Confirm this is acceptable.

For critical structural or high-strength bolted connections, consult a structural or mechanical engineer before substitution.
85. What documentation should I request when procuring galvanised fasteners?
For standard commercial applications:

• Delivery note with quantity and description.
• Galvanising certificate (confirming standard and coating thickness).

For critical structural, offshore, nuclear, or regulated applications:

• Full material traceability documentation.
• Material test certificate (EN 10204 3.1 or 3.2) showing chemical composition and mechanical properties.
• Heat treatment certificate (if applicable for high-strength fasteners).
• Hot-dip galvanising certificate with coating thickness measurements.
• Independent third-party inspection report (if specified).
• Compliance statement confirming conformity to project specifications.

Conclusion: Why Choose Hot-Dip Galvanising for Your Fastener Applications

Hot-dip galvanising remains the gold standard for long-term corrosion protection of steel fasteners in demanding applications. Its unique combination of exceptional durability, complete coverage, cathodic protection, and decades of proven performance makes it the first choice for:

• Infrastructure: Bridges, highways, railways, transmission towers, and public works.
• Marine and Offshore: Coastal structures, jetties, offshore platforms, and marine equipment.
• Industrial: Petrochemical plants, refineries, power generation, and heavy industry.
• Construction: Structural steelwork, building frames, cladding systems, and roofing.
• Agriculture: Livestock housing, grain silos, irrigation systems, and fencing.
• Utilities: Water treatment, wastewater, telecommunications towers, and energy infrastructure.

When you choose hot-dip galvanised fasteners from Trojan Special Fasteners Limited, you're choosing:

• Over 33 years of specialist fastener expertise serving UK and international markets.
• Comprehensive technical support from our experienced engineering team.
• Extensive stock holdings across metric, imperial, and unified size ranges.
• Expert oversize tapping services ensuring perfect fit with galvanised bolts.
• Full traceability and certification for critical and regulated applications.
• Competitive pricing and reliable delivery for projects large and small.

Contact Trojan Special Fasteners Limited Today

For technical advice, quotations, or to discuss your galvanised fastener requirements, contact our specialist team:

• Phone: 0121-789 8586
• Email: trojan.fasteners@gmail.com
• Website: www.TrojansF.co.uk
• Address: 18 Fortnum Close, Birmingham, B33 0LG

Trojan Special Fasteners Limited: Your Trusted Partner for Precision Fasteners and Expert Technical Solutions