TT-C-490: Pretreatment Types, Substrates, and Coating Rules

TT-C-490 is the federal specification governing chemical conversion coatings and pretreatments applied to metallic substrates before paint, varnish, enamel, or other organic coatings go on. It covers surface cleaning, the chemical treatment itself, and the tests that verify a pretreatment will resist corrosion and hold a primer. The current version is revision J, issued September 2024, and it applies to ferrous metals, aluminum alloys, and multi-metal assemblies.¹ Government contractors, military maintenance shops, and industrial coating applicators all work under it.

The Eight Pretreatment Types

TT-C-490 sorts pretreatments into eight types by chemistry. The type you use depends on the substrate and the level of corrosion protection the contract calls for.²

Type I: zinc phosphate, 150 to 500 mg/ft². The workhorse for military and heavy industrial use.
Type II: aqueous iron phosphate, minimum 35 mg/ft². A lighter treatment used where thick buildup would interfere with tolerances.
Type III: organic pretreatment.
Type IV: inorganic pretreatment.
Type V: medium-weight zinc phosphate, 500 to 1,100 mg/ft², for applications needing more protection than standard Type I.
Type VI: chemical conversion coatings for aluminum and aluminum alloys, processed under MIL-DTL-81706 and MIL-DTL-5541.
Type VII: anodic coating and electrolytic passivation.
Type VIII: metal-rich primers under MIL-PRF-32550.

Types III and IV currently have listings in the Qualified Products Database. Manufacturers pursuing approval for other types must qualify their products through the U.S. Army DEVCOM Army Research Laboratory.³

Substrate Classes

Class in TT-C-490 identifies the base metal, not the coating thickness. That trips people up.²

Class A: steel alloys consisting mostly of iron, excluding corrosion-resistant steel alloys.
Class B: aluminum alloys.
Class C: other alloys, multi-metal combinations of steel and aluminum, or other metallic substrates approved during qualification.

The class governs which qualified products are available and how a product appears in the QPD.

Surface Cleaning Requirements

Nothing else in the specification matters if the substrate isn’t clean first. TT-C-490 recognizes eight cleaning methods, and a facility can combine them to handle whatever contaminants are present. Other approaches are permitted if the contract allows them and the result meets the specification’s cleanliness thresholds.²

Method I: mechanical or abrasive cleaning, including blast cleaning, sanding, and grinding, per SSPC/AMPP standards.
Method II: solvent cleaning by immersion, spray, vapor, or hand wiping.
Method III: detergent cleaning by immersion, spray, ultrasonic, hot alkaline, or electrolytic methods.
Method IV: emulsion cleaning, with or without added water.
Method V: deoxidizing by chemical means.
Method VI: phosphoric acid cleaner, either detergent or solvent-type with detergent, which removes light rust while providing a mild etch.
Method VII: steam cleaning, with or without assisted pressure washing.
Method VIII: laser cleaning.

The final surface must be free of oils, salts, corrosion products, weld spatter, and other contaminants. Cleanliness is measured, not eyeballed. Adhesive patch testing cannot exceed 3 μg/cm² for immersed surfaces (such as tank interiors) or 5 μg/cm² for non-immersed surfaces. Conductivity testing caps allowable residue at 30 μS/cm for immersed surfaces and 70 μS/cm for non-immersed applications.² A part that looks clean can still fail these numbers, and that residual contamination is what causes coating failure later.

Application Forms and the 24-Hour Paint Window

Once the surface is clean, the conversion coating goes on using one of five application forms:²

Form 1: spray application.
Form 2: spray application followed by a rinse.
Form 3: immersion application.
Form 4: immersion application followed by a rinse.
Form 5: touch-up applications for localized repairs or areas missed during primary treatment.

Part size, geometry, and the contract terms drive the choice. Large structural components that don’t fit in a dip tank go through spray application; smaller fabricated parts get the uniform coverage that full immersion provides. Rinses after application remove unreacted chemicals that would otherwise interfere with the primer.

Drying has to happen promptly after the final rinse or the surface will flash rust. Paint must then be applied within 24 hours of pretreatment on thoroughly dried surfaces.² Miss that window and the coating may need to be redone. Where manual painting is involved, the work area needs at least 50 lumens per square foot (538 lux) so operators can catch holidays and runs as they work.

Coating Weight Acceptance

Coating weight is a primary acceptance criterion. Too light and the coating won’t protect against corrosion or hold a primer; too heavy and it becomes brittle or throws off dimensional tolerances. The ranges by type:²

Type I (zinc phosphate): 150 to 500 mg/ft². Immersion-applied coatings within this type tend to land in the 300 to 500 mg/ft² range because of longer contact time.
Type II (iron phosphate): minimum 35 mg/ft².
Type V (medium-weight zinc phosphate): 500 to 1,100 mg/ft².

Measurement is gravimetric: weigh the treated part, chemically strip the coating, and weigh again. Facilities typically test sample pieces from each production batch rather than every part.

Stress Relief for High-Strength Steel

Any part with a surface or through hardness of Rockwell C 39 or greater must be stress-relieved before phosphating. The treatment: heat to 350–400 °F (177–204 °C), or 50 °F below the tempering temperature if that is lower, for a minimum of one hour per inch of thickness, with at least a half hour for parts thinner than half an inch.² Skipping this risks hydrogen embrittlement, which can cause the part to fail under load without warning.

Hexavalent Chromium Is Prohibited by Default

Older versions of TT-C-490 permitted chromate-based rinses and sealers containing hexavalent chromium (Cr6+), a known carcinogen. The current specification prohibits hexavalent chromium on coated end items for all types and classes unless the contract or engineering drawing explicitly authorizes it.¹ This is a default prohibition. Facilities that once used chromic acid final rinses to boost corrosion resistance need approved alternatives or written contract authorization to continue.

Qualified Products Database

Products used under TT-C-490 have to appear on the Qualified Products Database maintained by the Defense Logistics Agency. Listings are organized by type, class, and form, and the database updates as manufacturers complete qualification testing.³ Using an unlisted product is a noncompliance even if the chemistry looks identical to a qualified one. Qualification runs through the U.S. Army DEVCOM Army Research Laboratory, and additional requirements can apply depending on end use.

Quality Evidence and Testing

TT-C-490 treats Objective Quality Evidence as an ongoing obligation, not a one-time qualification hurdle. Facilities must demonstrate control through certifiable process checks aligned with ISO 9001 or ISO 17025, backed by continuous monitoring data.⁴ Records need to cover bath chemistry, temperature logs, coating weight results, and cleanliness testing.

Performance verification typically uses two standard tests. Adhesion is evaluated with the cross-hatch tape test under ASTM D3359, which scores how well the coating resists being pulled off after a grid is cut through it.⁵ Corrosion resistance is assessed with salt spray exposure under ASTM B117, in a chamber held at 95 °F with a continuous 5% sodium chloride mist. Failure shows up as blistering, rust creepage from scribe marks, or loss of adhesion after a contract-specified number of hours.

Failing acceptance testing isn’t only a scrap problem. A facility that knowingly delivers nonconforming goods under a government contract faces False Claims Act exposure of treble damages plus per-claim civil penalties.⁶ Accurate recordkeeping carries as much weight as the pretreatment itself.

Environmental and Workplace Safety

Phosphating and related processes generate sludge and spent bath solutions that often qualify as hazardous waste under EPA’s Resource Conservation and Recovery Act. Electroplating sludge classified as F006 is a common example. Generators may accumulate F006 waste on-site for up to 180 days (or 270 days in certain circumstances) without a RCRA permit, but only if the waste is destined for metals recovery, pollution prevention practices are in place, and no more than 20,000 kilograms accumulate at one time.⁷ Waste not headed for metals recovery does not qualify for the extended period.

OSHA permissible exposure limits apply to the chemicals in routine use. Phosphoric acid (Method VI cleaning) has an 8-hour time-weighted average of 1 mg/m³. Sodium hydroxide, a common alkaline cleaner component, carries a ceiling limit of 2 mg/m³. Sulfuric acid, sometimes present in deoxidizing baths, is limited to 1 mg/m³.⁸ Facilities operating with hexavalent chromium under a contract exemption face the tightest limit, 0.005 mg/m³. Ventilation, personal protective equipment, and air monitoring are practical necessities in any shop running these processes.

1
ASSIST Quick Search. Chemical Conversion Coatings and Pretreatments for Metallic Substrates (Base for Organic Coatings)
2
ASSIST Quick Search. Chemical Conversion Coatings and Pretreatments for Metallic Substrates – TT-C-490H
3
DLA QPD Search. QPL-TT-C-490 Qualified Products Database
4
Defense Technical Information Center. TT-C-490 – Implementing Alternatives Through Specifications
5
ASTM International. ASTM D3359-23 – Standard Test Methods for Rating Adhesion by Tape Test
6
Office of the Law Revision Counsel. 31 USC 3729 – False Claims
7
Environmental Protection Agency. Changes to the Final Rule and the Regulatory Impact Analysis
8
Occupational Safety and Health Administration. Permissible Exposure Limits – OSHA Annotated Table Z-1