Chemical Conversion Coatings on Aluminium & Aluminium Alloys
Specifications
Type:
Type I ~ composition containing hexavalent chromium.
Type II ~ compositions not containing hexavalent chromium. (non-hexavalent also known as trivalent);
Chemical used: Surtac 650
Class:
Class 1A – For maximum protection against corrosion, painted or unpainted.
Class 3 – For protection against corrosion where low electrical resistance is required.
Federal Standards
FED-STD-141 ~ Paint, Varnish, Laquer and related materials. Method of inspection, sampling and testing.
Department of Defence Spesifications.
 MIL-PRF-23377 – Primer coatings: epoxy, high solids.
 MIL-DTL-81706 – Chemical conversion Material for coating aluminium and aluminium alloy.
 MIL-PRF-85582 – Primer Coatings: epoxy, waterborne.
American Society for testing and materials (ASTM)
 ASTM-B117 ~ Salt spray (fog) apparatus, operating.
 ASTM-D3359 ~ Adhesion by tape test, Measuring.
American Soceity for Quality (ASQ)
ASQ-Z1.4 ~ Procedures, sampling and tables for inspection by attributes.
Requirements:
1. Materials
- The material used to produce a chemical conversion coating shall be approved for the selected
types, class, forms and application method in accordance for listing on the qualification
requirements.
2. Cleaning
- Prior to coating, the base metal shall be mechanically or chemically cleaned. Test by using (water
break free test) after rinsing.
- Abrasives containing iron such as (steel wire are prohibited for all cleanings.
- Treated parts that have become soiled shall be cleaned with materials that will remove the soil
without damaging the base metal, part or the conversion coating.
- Chemical conversion coating shall be applied “non-electrotically” by spray, brush or immersion
after al heat treatment and mechanical operations such as forming, perforating, machining,
brazing and welding have been complete.
- Items containing non-aluminium parts that may be attacked, embritted or damaged.
- In any way by the conversion coating process shall not be coated as assemblies unless the nonaluminum parts are masked covered.
3. Touch Up
Damaged area must be touched up with material approved for the applicable type, class, form
and method.
Area touched up must not greater than 5% of the item surface area.
4. Appearances
- Must be continuous and visibly discernible in daylight.
- Must free from powdery areas or loose coating, voids, scratches, flaws and other defects or
damages that will reduce the serviceability of parts or detrimental to the protective value and paint
bonding characteristics.
5. Performance Characteristics
5.1 – Corrosion Resistance
- After 168 hours of exposures to the 5% salt spray test, the test speciments will be treated with the
applicable class of coating shall meet the corrosion resistance requirements.
-
Conditions:
1. Not more than 5 isolated spots or pits.
2. Not larger than 0.031 inch (diameter) per speciments.
3. Areas within 0.25 inch from the edge.
4. Identification markings.
5. Holding points during processing or salt spray exposure shall be excluded.
5.2 - Paint Adhesion
- When the production paint system applied to the test specimens, no intercoat separation shall
occur between the paint system and the conversion coating or between the conversion coating
and the base metal
- If the conversion coated parts do not require painting for end use, the paint adhesion can be
omitted if specifically authorized by the acquisition activity.