1.1 These practices cover two classes of treatment for preparation of magnesium alloy surfaces for painting, as follows:
Class I-Chemical Treatments.
Class II-Anodic Treatments.
In general, the latter treatments are the more protective of the two classes. Mechanical (abrasive) treatments, solvent cleaning, alkaline solution treatments, and acid pickles not resulting in protective conversion coatings are suitable preliminary treatments only for metal to be exposed under mildly corrosive (indoor) exposures. When a high degree of corrosion protection and paint adhesion are desired, as in many outdoor environments, surface preparation by one of the above conversion-coat classes is necessary. The hexavalent chromium based methods given are not recommended as hexavalent chromium is a known carcinogen.
Note 1: Testing of Coatings-Quality control tests of coatings are frequently desirable, and these generally consist of exposures, with or without paint, to salt spray, humidity, or natural environments, with suitable procedures for assessing the degree of breakdown suffered after fixed time intervals. It is recommended that quality control tests of coatings shall be made as far as possible with high-purity material (for example AZ31A alloy),2 the inherent corrosion rate of which is relatively consistent from batch to batch and that precautions shall be taken to remove surface contamination before coatings are applied. Such contamination shall be removed by acid pickling to a depth of at least 0.001 in. (25 μm) per side.
Product Details
- Published:
- 10/01/2018
- Number of Pages:
- 8
- File Size:
- 1 file , 120 KB
