Metal Surface Treatment, Conversion Coatings and Plating: the practitioner

the practitioner: "We've Always Done It This Way"

the practitioner's plant had been running the same die setup for years. Carbon steel dies, oil-quenched and tempered, with a light phosphate coating. It worked fine — until the customer changed the specification from mild steel blanks to high-strength, low-alloy steel.

Nobody thought to revisit the surface treatment.

This is the mistake that costs manufacturing operations millions every year. The base metal gets all the attention — alloy selection, heat treatment, hardness testing — while the surface, the actual interface where work happens, gets treated as an afterthought.

Here's the reality you need to internalize:

Every manufactured part lives or dies at its surface. Corrosion attacks from the outside in. Wear happens at the contact interface. Fatigue cracks initiate at surface imperfections. The coating, plating, or surface treatment you choose isn't decoration — it's the first and last line of defense.

The world of material treatment is vast. It includes:

• Conversion coatings (phosphate, chromate, oxide films)
• Electroplating (chrome, nickel, cadmium, zinc, gold, silver, copper, tin)
• Electroless plating (nickel-phosphorus, nickel-boron)
• Anodizing (chromic, sulfuric, hard coat)
• Hard facing (weld-deposited wear surfaces)
• Flame and plasma spraying (thermal spray coatings)
• Laser surface treatments (hardening, cladding, marking)
• Etching and coloring (chemical surface modification)
• Chemical films and passivation (corrosion inhibition)

Each has a purpose. Each has specifications. Each has failure modes that will cost you dearly if you choose wrong.

Let's walk through every one of them — the way the practitioner did — starting with the foundations.

Act I: The Foundation — Conversion Coatings and the Chemistry of Surfaces

What Conversion Coatings Actually Are

When the practitioner started investigating her die failure, the first thing she revisited was the phosphate coating that had been specified for years.

Conversion coatings are thin, adherent chemical compounds produced on metallic surfaces by chemical or electrochemical treatment. Unlike plating, which adds material to the surface, conversion coatings transform the surface itself. The metal reacts with a chemical solution, and the outermost layer of the base metal becomes an integral, insoluble, protective film.

These coatings are divided into two basic systems:

• Oxides or mixtures of oxides with other compounds
• Chromates or phosphates

They serve three primary functions:

1. Corrosion protection — a passive barrier against environmental attack
2. Paint adhesion — a chemically roughened surface that grips primers and topcoats
3. Decorative finish — inherent color plus the ability to absorb dyes and sealants

The Four-Step Conversion Coating Process

Every conversion coating follows the same fundamental sequence:

Application methods include immersion, spraying, or brushing — chosen based on part size, geometry, and production volume.

Phosphate Coatings: The Workhorse of Ferrous Metal Treatment

Phosphate coatings are applied to iron and steel parts by reacting them with a dilute solution of phosphoric acid and other chemicals. The surface metal converts into an integral, mildly protective layer of insoluble crystalline phosphate.

Small items are coated in tumbling barrels. Large items are spray-coated on conveyors.

Three types of phosphate coatings dominate industrial use:

Critical limitation: Stainless steels and certain alloy steels cannot be phosphated. Most cast irons and alloy steels accept coating with varying degrees of difficulty depending on alloy content.

Light Phosphate Coating — TT-C-490D

This specification covers both cleaning methods and pretreatment processes:

Cleaning Methods:

Pretreatment Coating Types:

Heavy Phosphate Coating — DOD-P-16232-F

The heavy coatings are designed for holding and retaining supplemental coatings that provide the major corrosion resistance:

Class designations for both types:

These coatings range from gray to black and are specified for medium and low alloy steels.

Passivation: Making Stainless Steel Actually Stainless

When the practitioner's investigation expanded to the stainless steel components in her tooling, she discovered that several parts had been skipping their passivation step — a cost-cutting measure that was now causing pitting corrosion.

Passivation (QQ-P-35C) improves the corrosion resistance of parts made from:

• Austenitic corrosion-resistant steels (200 and 300 series)
• Ferritic corrosion-resistant steels (400 series)
• Martensitic corrosion-resistant steels (400 series)
• Precipitation-hardened corrosion-resistant steels

Note: 440C grades may be exempt from passivation at the discretion of the procuring activity.

Passivation solution types:

Passivation of Copper

The blue-green patina that forms on copper alloys during atmospheric exposure is itself a passivated film — it prevents further corrosion. This patina can be produced artificially using a solution of:

• Ammonium sulfate: 6 lb
• Copper sulfate: 3 oz
• Ammonia (technical grade, 0.90 specific gravity): 1.34 fl oz
• Water: 6.5 gallons

Applied as a fine spray to a chemically cleaned surface and allowed to dry between each of five or six applications, a patina somewhat bluer than natural begins to develop in about 6 hours.

For small copper parts, an immersion or brushing solution consists of (by weight): copper, 30; concentrated nitric acid, 60; acetic acid (6%), 600; ammonium chloride, 11; ammonium hydroxide, 20. The copper is dissolved in the nitric acid before adding remaining chemicals, and the solution must stand for several days before use. A linseed oil coating is applied to treated parts.

Coloring of Metals: When Appearance Is the Specification

the practitioner didn't deal directly with decorative finishes, but her company's product line included architectural hardware where color was the deliverable. Understanding how metals are colored gave her insight into the chemical behavior of every surface she worked with.

Metals are colored for three reasons:

1. To enhance appearance
2. To produce an undercoat for organic finishes
3. To reduce light reflection

Coloring of Copper Alloys

The final color depends on four variables: base metal composition, coloring solution composition, immersion time, and operator skill. Cleaning is critical — nitric and sulfuric acid solutions remove oxides and activate the surface.

For alloys with ≥ 85% copper:

For alloys with < 85% copper:

Post-treatment: Scratch brush to remove excess deposits. Color contrast can be achieved by brushing with fine pumice slurry, hand rubbing with abrasive paste, mass finishing, or buffing to remove color from highlights. Clear lacquer for outdoor parts; indoor parts often need no additional protection.

Coloring of Iron and Steel

Black oxide coatings are applied by immersing steel parts in a boiling solution of sodium hydroxide and mixtures of nitrates and nitrites. These coatings serve as paint bases and, in some cases, as final finishes. When impregnated with oil or wax, they provide fairly good corrosion resistance at relatively low cost compared to other coatings.

Anodizing Aluminum Alloys: Building an Oxide Armor

Anodizing was the process that opened the practitioner's eyes to the concept of growing a protective layer from the base metal itself, rather than depositing one on top.

In the anodizing process:

1. The aluminum object is immersed as the anode in an acid electrolyte
2. Direct current is applied
3. Oxidation of the surface occurs, producing a greatly thickened, hard, porous film of aluminum oxide
4. The object is then immersed in boiling water to seal the porosity and render the film impermeable
5. Before sealing, the film can be colored by impregnation with dyes or pigments

The three principal anodizing processes:

Chemical Conversion Coatings for Aluminum

These are adherent surface layers of low-solubility oxide, phosphate, or chromate compounds produced by the reaction of the metal surface with suitable reagents. They are much thinner and softer than anodic coatings but less expensive and serve as an excellent paint base.

Magnesium Alloys

Chemical treatment provides a paint base and improves corrosion resistance. The popular conversion "dip" coatings are chrome pickle and dichromate treatments — very thin. Anodic coatings are thicker, harder, and after sealing give equal corrosion protection, though painting is still desirable.

Titanium Alloys

Chemical conversion coatings improve lubricity by acting as a base for lubricant retention. The coatings are applied by immersion, spraying, or brushing. A popular bath is an aqueous solution of phosphates, fluorides, and hydrofluoric acid, producing a coating of titanium and potassium fluorides and phosphates.

Anodize (Chromic and Sulfuric) — MIL-A-8625F

Conventional Types I, IB, and II anodic coatings improve surface corrosion protection under severe conditions or serve as a base for paint systems.

Class 1: Non-dyed | Class 2: Dyed (color specified on contract)

Always specify the class of anodic coating and any special sealing requirements.

Hard Anodize — MIL-A-8625F (Type III)

Color varies from light tan to black depending on alloy and thickness. Can be dyed in darker colors.

Key characteristics:

• Coating penetrates base metal as much as it builds up on the surface — thickness includes both buildup and penetration
• Provides a very hard ceramic-type coating
• Abrasion resistance varies with alloy and thickness
• Good dielectric properties

Critical specification details:

• Thickness: Specified on contract or drawing. Default nominal: 0.002 in. Range: 0.0005–0.0045 in.
• Class 1: Not dyed or pigmented
• Class 2: Dyed, color specified on contract
• Do NOT seal coatings where maximum abrasion/wear resistance is the primary function
• Do seal (boiling deionized water or hot 5% sodium dichromate) for exterior applications requiring corrosion resistance where reduced abrasion is acceptable
• Thick coatings (> 0.004 in.) will tend to break down sharp edges
• "Flash" hard anodize may substitute for conventional anodize for corrosion resistance and may be more economical when combined with other hard anodized areas

Black Chrome — MIL-C-14538C

A hard, non-reflective, abrasion-, heat-, and corrosion-resistant coating approximately 0.0002 in. thick.

• Color: Dull dark gray, approaching black; may be waxed or oiled to darken
• Corrosion protection: Limited; add nickel underplate for increased protection
• Throwing power: Poor — conforming anodes necessary for intricate shapes
• Process sequence: Apply after heat treating and all mechanical operations
• Stress relief: Steel parts > 40 Rc must be baked 1+ hour at 300–500°F prior to plating
• Post-plating bake: 375°F ± 25°F for 3 hours

Black Oxide Coating — MIL-C-13924C

A uniform, mostly decorative black coating for ferrous metals used to decrease light reflection.

• Corrosion protection: Very limited, under mild conditions only
• Supplementary treatment: Should normally be applied (oil, wax, lacquer)
• Best application: Moving parts that cannot tolerate dimensional change of more corrosion-resistant finishes

Oxidizing method selection:

Cadmium Plating — QQ-P-416F

Cadmium plating must be smooth, adherent, uniform in appearance, free from blisters, pits, nodules, and burning.

Critical restriction: Brightening agents in the plating solution are prohibited on components with heat treatment of 180 ksi minimum tensile strength (or 40 Rc) and higher unless specifically authorized.

Baking: On Types II and III, baking must be done prior to application of supplementary coatings.

Vacuum Cadmium — MIL-C-8837B

Provides corrosion resistance to ferrous parts free from hydrogen contamination and possible embrittlement. Recommended on steels with a strength of 2.2 × 10⁵ psi or above.

Process sequence:

1. Coating is applied after all machining, brazing, welding, and forming is completed
2. All steel parts must be stress relieved by baking at 375°F ± 25°F for 3 hours if suspected of having residual tensile stresses
3. Immediately prior to coating, lightly dry abrasive blast areas to be coated

Temperature limit: Cadmium coating shall not be used if in-service temperature reaches 450°F.

Salt spray test (Type II): 96 hours.

Chrome Plating — QQ-C-320B

Excellent hardness, wear resistance, and erosion resistance. Low coefficient of friction, heat-resistant, and can be rendered porous for lubrication purposes.

Baking requirements (all coated steel parts Rc 36 and higher):

The Science of Chromium Plating — Deep Dive

Chromium plating is an electrolytic process of depositing chromium on metals for corrosion protection or to increase surface-wearing qualities. The technology has proven especially valuable for precision gages — chromium-plated plug and ring gages wear dramatically longer, and when worn, the chromium can be stripped, replated, and reground to size.

Tool applications with proven performance gains:

• Drills, taps, reamers, files, broaches, tool tips, saws, thread chasers
• Dies for stamping, drawing, hot forging, die casting, and plastics molding

Plating thickness range: 0.0001 to 0.001 or 0.002 in. Thicker platings are used to build up undersize tools such as taps and reamers.

Standard chromium plating procedure:

1. Degrease with solvent
2. Mount tools on racks
3. Clean in anodic alkali bath at 82°C for 3–5 minutes
4. Rinse in boiling water
5. Immerse in 20% hydrochloric acid solution for 2–3 seconds
6. Rinse in cold water
7. Rinse in hot water
8. Etch in reverse-current chromic acid bath for 2–5 minutes
9. Place work immediately in chromium plating bath
10. Remove hydrogen embrittlement (if necessary) by immersing plated tools for 2 hours in oil bath at 177°C

Friction properties of chromium:

Values apply to highly polished bearing surfaces.

Chemical resistance: Chromium resists attack by almost all organic and inorganic compounds except muriatic and sulfuric acids. Melting point: 2930°F. Remains bright up to 1200°F. Above 1200°F, a light adherent oxide forms that does not readily detach, making chromium suitable for high-temperature applications even above 2000°F.

Copper Plating — MIL-C-14550B

Good corrosion resistance when used as an undercoat. Multiple process variants exist for specific purposes: brightness improvement, high-speed electroforming, and fine grain for preventing case hardening.

Hydrogen embrittlement relief: All steel parts ≥ Rc 35 must be baked at 375°F ± 25°F for 24 hours, within 4 hours after plating. Springs and flexure parts must not be flexed prior to baking.

Nickel Plating — QQ-N-290A

There is a nickel finish for almost any need. Nickel can be deposited soft, hard-dull, or bright depending on the process and plating conditions.

• Hardness range: 150–500 Vickers
• Color: Similar to stainless steel, or dull gray (almost white)
• Corrosion resistance: Function of thickness
• Thermal expansion: Low coefficient

Critical restriction: Steel parts with tensile strength of 220,000 psi or greater shall not be nickel plated without specific approval of the procuring agency.

Electroless Nickel — AMS 2404C, AMS 2405B, AMS 2433B

Provides a hard, ductile, wear-resistant, and corrosion-resistant surface for operation up to 1000°F and provides uniform build-up on complex shapes — a major advantage over electrolytic plating.

Preparation: Parts > Rc 40 that have been machined or ground after heat treatment must be stress-relieved before cleaning and plating.

Post-treatment baking:

Gold Plating — MIL-G-45204C

• Color: Yellow to orange depending on proprietary process
• Finish: Matte to bright depending on basis metal
• Properties: Good corrosion resistance, high tarnish resistance, low contact resistance, excellent electrical conductivity, excellent solderability

For soldering: Thin, pure, soft gold coating preferred. Thickness: 0.00005 in. minimum, 0.00010 in. maximum.

Critical rule: Gold over silver underplate combinations are excluded from electronics hardware unless required by item specification. When gold is applied to brass, bronze, or beryllium copper (or copper plate/strike), a nickel antidiffusion underplate must be applied.

Silver Plating — QQ-S-365D

Silver has increasing use in both decorative and engineering fields, including electrical and electronic applications.

• Appearance: White matte to very bright
• Corrosion resistance: Good, depending on base metal; will tarnish easily
• Hardness: 90–135 Brinell depending on process
• Solderability: Excellent, but decreases with age
• Conductivity: Best conductor of electricity
• Lubricity: Excellent smear characteristics for antigalling uses on static seals, bushings, etc.

Stress relief: Steel parts at minimum 375°F ± 25°F prior to cleaning and plating if they contain or are suspected of having residual tensile stresses.

All types and grades: Minimum thickness 0.0005 in. unless otherwise specified.

Tin Plating — MIL-T-10727C

Two coating methods: electrodeposited (per ASTM B545) and hot dipped.

• Color: Gray-white in plated condition
• Properties: Soft but very ductile, good corrosion resistance, excellent solderability
• Limitation: Not good for low-temperature applications

For a bright finish in lieu of fused tin, specify Bright Tin plate — thickness can exceed that of fused tin with excellent corrosion resistance and solderability.

Zinc Plating — ASTM B633

Electrodeposited zinc coatings for iron or steel articles to protect from corrosion.

Critical restrictions:

• High-strength steels (tensile strength > 1700 MPa) shall not be electroplated
• Stress relief: All parts ≥ 1000 MPa tensile at minimum 190°C for 3+ hours before cleaning and plating
• Hydrogen embrittlement relief: All electroplated parts ≥ 1200 MPa must be baked at 190°C for 3+ hours within 4 hours after electroplating

Rhodium — MIL-R-46085B

Metallic, similar to stainless steel in color. Excellent corrosion and abrasion resistance, almost as hard as chromium, high reflectivity. Thicker coatings are very brittle.

• Pre-plating bake: Parts ≥ Rc 33 at 375°F for 3 hours
• Post-plating bake: Parts ≥ Rc 40 at 375°F for 3 hours within 4 hours after plating

Palladium — MIL-P-45209B

A gray, dense deposit good for undercoats with good wear characteristics, corrosion resistance, catalytic properties, and conductivity.

• Thickness: 0.00005 in. unless otherwise specified
• Post-plating: Steel springs and other parts subject to flexure with hardness > Rc 40 must be heated to 375°F ± 25°F for 3 hours

Chemical Films — MIL-C-5541E

Produce coatings ranging from clear to iridescent yellow or brown. Clear coatings may cause inspection difficulties due to invisibility.

Chemical Finish: Black — MIL-F-495E

A uniform black corrosion retardant for copper.

• Abrasion resistance: None
• Application: Blacken color and reduce gloss on copper-alloy surfaces (not for food service or water supply)
• Also used as: Base for lacquer, varnish, oil, and wax

Electropolishing (No MIL-SPEC)

Electrolytically removes or diminishes scratches, burrs, and unwanted sharp edges from most metals. Finishes from satin to mirror-bright are produced by controlling time, temperature, or both.

• Typical thickness loss: 0.0002 in.
• Not recommended for close-tolerance surfaces

Solid Film Lubrication — MIL-L-46010D

Three types of heat-cured solid film lubricants to reduce wear and prevent galling, corrosion, and seizure. For use on aluminum, copper, steel, stainless steel, titanium, chromium, and nickel-bearing surfaces.

Thickness: 0.008–0.013 mm (no single reading < 0.005 mm or > 0.018 mm).

Color 1: Natural product color | Color 2: Black

Magnesium Process — MIL-M-3171C