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A Complete Guide to Phosphate & Chemical Blacking Finishes

Understanding Conversion Coatings for Steel Components

When selecting a finish for steel components, the terms 'phosphating', 'Parkerizing', and 'chemical blacking' are often used. While all are types of conversion coatings – meaning they chemically alter the surface of the steel rather than adding a layer on top – they represent distinct processes with very different properties and applications.

Understanding these differences is critical for specifying the correct finish for corrosion resistance, wear protection, or purely aesthetic purposes. This guide provides a detailed breakdown of each process.

Phosphate Conversion Coatings (Phosphating)

Phosphating is a chemical process that transforms the surface of ferrous metals into a non-metallic, crystalline layer of insoluble phosphate salts. This new layer is an integral part of the metal itself, providing excellent adhesion and a microporous structure that is perfect for absorbing oils and other sealants.

The process involves immersing the cleaned steel part into a heated, acidic solution of phosphate salts. There are three primary types.

A Deep Dive: Manganese Phosphate (Parkerizing)

When it comes to protecting metal components from wear and corrosion, the choice of surface finish is critical. One of the most durable and trusted methods is Parkerizing.

At Trojan Special Fasteners Ltd, we specialise in providing high-performance fasteners for demanding applications. We often supply parts that must conform to precise engineering specifications, and Parkerizing is a finish that can be supplied to meet some of the most stringent standards.

What is Parkerizing?

Parkerizing is the trade name for a specific type of manganese phosphate coating. Think of it like "Hoover" for vacuum cleaners—"Parkerizing" is a brand name that has become synonymous with the process itself.

In technical terms, it is a chemical conversion coating applied to steel and iron components. The process converts the surface of the metal into a non-metallic, crystalline layer of manganese phosphate. This layer provides a rugged, dark grey to black matte finish that is highly resistant to corrosion and wear.

At Trojan Special Fasteners Ltd, when we refer to Parkerizing, we are specifically talking about Black Manganese Phosphating, which is renowned for its superior performance properties.

How does the Parkerizing (Manganese Phosphating) process work?

The process is a multi-stage chemical treatment which generally involves these key steps:

• Thorough Cleaning: The part is completely cleaned of all contaminants like oil, grease, and rust.
• Immersion Bath: The clean metal part is then fully submerged in a hot, acidic solution of phosphoric acid and manganese salts.
• Chemical Reaction: The solution reacts with the steel, etching the surface and depositing a uniform, crystalline layer of manganese phosphate.
• Rinsing: The component is removed and thoroughly rinsed to neutralise any remaining acid.
• Sealing: Finally, the Parkerized surface is sealed, typically by immersing it in a special oil. The microporous structure of the coating is excellent at absorbing this sealant, which significantly enhances its protective properties.

What are the main benefits of a Parkerized finish?

Black Manganese Phosphating is chosen for its unique combination of protective qualities.

1. Excellent Corrosion Resistance: The non-metallic phosphate layer, combined with the sealing oil, creates a formidable barrier against moisture and oxidation.
2. Superior Wear Resistance: The hard, crystalline structure is highly resistant to scratching and scuffing, extending the service life of components.
3. Improved Lubricity & Anti-Galling: The coating's ability to hold oil provides constant self-lubrication, which is essential for threaded fasteners to prevent galling under pressure.
4. Reduces Light Glare: The dark, matte finish is non-reflective, a key reason for its widespread use in military applications.

Where are Parkerized components typically used?

Due to its rugged and durable nature, Parkerizing is specified across many demanding industries, including fasteners, military & defence, automotive, aerospace, and oil & gas.

A Deeper Look: Zinc Phosphate

This is the most common type of phosphating for corrosion resistance, especially as a base for painting.

• The Process: Uses a solution of zinc salts to create a phosphate layer. The process can be controlled to produce different crystal sizes, from microcrystalline for the smoothest paint finishes to heavier macrocrystalline structures for maximum oil retention.
• Appearance: Light to medium grey with a fine, smooth crystalline structure.
• Key Properties:
  • Excellent Corrosion Resistance: Provides a robust barrier against rust. Unlike manganese phosphate, its primary strength is preventing under-paint corrosion creep, where rust spreads underneath a paint film from a scratch.
  • Superb Paint Adhesion: This is the main reason for its use in the automotive industry. The crystal structure acts as a perfect primer, creating a strong mechanical bond for paints, e-coats, powder coatings, and other organic finishes.
  • Good Oil Retention: While not as effective as manganese phosphate for wear resistance, it still holds oil well, providing good lubricity for applications like cold forming or for threaded fasteners.
• Applications: Automotive bodies and components ('e-coat' pretreatment), stampings, fasteners requiring a painted or organic topcoat, and as a corrosion-resistant finish on its own when oiled.

Iron Phosphate

This is the most economical phosphating process, used primarily as a basic pretreatment for painting.

• The Process: The simplest process where the steel part itself is the source of the iron, creating a very thin iron phosphate layer. It is often applied via spray rather than immersion.
• Appearance: Can range from a clear or bluish iridescent sheen to a light grey.
• Key Properties:
  • Good Paint Adhesion: Provides a clean, etched surface that improves paint bonding.
  • Minimal Corrosion Resistance: Offers only short-term protection on its own.
• Applications: Appliance casings, office furniture, and general sheet metal parts that will be painted or powder coated for indoor use.

Relevant Standards for Phosphate Coatings

Yes, there are several key international standards for phosphating. It's important to understand the different classification systems they use.

BS EN ISO 9717:2017 - The International Standard

This is the primary British and International standard for "Phosphate conversion coatings on metals". It classifies coatings by their chemical type and coating mass (how thick the layer is).

• Mn: A heavy Manganese Phosphate coating (Parkerizing).
• Zn: A heavy Zinc Phosphate coating.
• ZnCa: A Zinc-Calcium Phosphate coating, often used for a finer crystal structure.
• Fe: An Iron Phosphate coating.

MIL-DTL-16232G - The US Military Standard

This is a very common standard used worldwide due to its clear classification. The "Type" you referred to comes from this specification.

• Type M: Specifies a Manganese Phosphate coating, used for wear resistance. This is what is commonly meant by Parkerizing.
• Type Z: Specifies a Zinc Phosphate coating, used as a paint base and for corrosion resistance.

DEF STAN 03-11 - The UK Defence Standard

This is the UK's Ministry of Defence standard for the "Phosphate Treatment of Iron and Steel" and is functionally similar to the US military standard.

Chemical Blacking (Black Oxide) in Detail

Chemical Blacking, also known as black oxide or chemi-blacking, is a completely different conversion coating. It is an alkaline process that does not use phosphate salts.

• The Process: This is a highly controlled, multi-stage immersion process.
  • # Cleaning: Parts are first placed in a hot alkaline cleaner to remove all oils and grease.
  • # Rinsing: The parts are thoroughly rinsed to remove the cleaning solution.
  • # Acid Activation: A brief dip in an acid bath (like hydrochloric acid) slightly etches the surface to remove rust and prepare it for the reaction.
  • # Blackening: The parts are immersed in a boiling bath (around 145°C) of alkaline oxidizing salts (typically sodium hydroxide and nitrates). This process converts the surface of the steel into magnetite (Fe₃O₄), a stable black oxide of iron.
  • # Sealing: After final rinsing, the parts, with their newly formed porous oxide layer, are immediately sealed with a rust-preventative oil, wax, or dewatering fluid. This final step is critical for the finish's performance.
• Appearance: A deep, uniform, aesthetically pleasing black. The final finish can range from matte to glossy depending on the surface finish of the original part and the type of sealant used.
• Key Properties:
  • Minimal Dimensional Change: The black oxide layer is extremely thin (typically 1 micron or less). This makes it the perfect finish for high-precision components, threaded parts, and assemblies where dimensional tolerances are critical and cannot be altered by a thicker coating.
  • Mild Corrosion Resistance: The corrosion resistance comes almost entirely from the final oil or sealant. A light oil may only provide 2-8 hours of salt spray resistance, while a heavy wax can provide up to 100 hours. It is primarily suitable for indoor environments.
  • Decorative Finish: Provides an attractive and non-reflective black finish without chipping, peeling, or rubbing off, as it is an integral part of the metal.
• Applications: Tooling, gauges, firearm components, engine parts, gears, and fasteners where maintaining precise dimensions is more critical than high corrosion resistance.

Relevant Standards for Chemical Blacking

1. BS EN ISO 11408:2009: The primary British and International standard for "Black oxide coatings on iron and steel".
2. MIL-DTL-13924D: A key US military specification for "Coating, Oxide, Black, for Ferrous Metals". It defines different classes based on the type of sealant used.