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9/8/98 AC 43.13-1B SECTION 3. CORROSION PROTECTION MEASURES FOR BASIC MATERIALS 6-29. GENERAL. In the repair of aircraft, apply corrosion proofing of the same type or equivalent to that originally applied unless the repair would result in increased susceptibility to corrosion, in which case use additional cor rosion protection measures. The following is a list of the most commonly-used corrosion- proofing techniques. 6-30. ANODIZING AND RELATED PROCESSES. In anodizing, aluminum alloys are placed in an electrolytic bath causing a thin film of aluminum oxide to form on the surface of the aluminum. This is resistant to corrosion and affords a good paint base. However, other processes, which do not provide as good a cor rosive protection as anodizing, are good paint bases. The processes are: a. Alkaline cleaning followed by chromic acid dip; high-strength steel parts. High-strength nuts and bolts are susceptible to failure from hydro gen embrittlement. Because of the potential failures of embrittled parts, careful control over the heat treatment, grinding, preplate cleaning, plating, and post-plate baking of high-strength parts is necessary. 6-32. PHOSPHATE RUST-PROOFING. This process is commercially known as Parkerizing, Bonderizing, Granodizing, etc. The coating placed on the part is used to pro tect steel parts after machining and before painting. 6-33. CHROME-PICKLE TREATMENT. Magnesium parts which have been immersed or brushed with a solution of nitric acid and sodium dichromate will be protected for tem porary storage. The coating will also serve as a bond for subsequent organic finishes. Sealed chrome-pickle treatment is used on magnesium parts for long term protection. Diluted chro mic acid is a touch-up treatment. It is less critical to apply and can be applied over previ ously-applied thin chromate films. 6-34. DICHROMATE TREATMENT. This treatment consists of boiling magnesium parts in a solution of sodium dichromate. It provides good paint base and protective quali ties on all standard wrought magnesium alloys except the magnesium-thorium alloys. Acid pickling of the magnesium surface prior to ap plication of the dichromate treatment is re quired if maximum corrosion resistance of the finish is expected. 6-35. STANNATE IMMERSION TREAT- MENT. Stannate immersion treatment deposits a layer of tin. It is a protective paint base for magnesium alloy parts which contain inserts and fasteners of a dissimilar metal such as b. c. Alcoholic phosphoric acid cleaner; and Alkaline dichromate treatment. 6-31. PLATING. Steels are commonly platedwithothermetalstopreventcorrosion. Plating is accomplished by placing the article in an electrolytic bath. Metals used in plating vary in the corrosion protection they afford steel. For instance, in platings that corrode be fore steel, such as zinc or cadmium, slight breaks or cracks throughout the plating will not result in rusting of the exposed steel. With the surface metal corroded, the steel is pro tected. However, when the steel corrodes faster than the plate metal, such as chromium, the amount of protection depends on the tight ness of the plating. Post-plate bake treatment to relieve hydrogen embrittlement is a neces sary part of replating procedures for Par 6-29 Page 6-11PDF Image | AFS-640
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