Cronak process

The most commonly used chromate passivation process is the Cronak process developed by the New Jersey Zinc Co. in 1936, in which the parts are immersed for 5-10 s in a solution containing 182 g/1 sodium dichromate and 6ml/l sulphuric acid. A golden irridescent film is formed on the zinc or cadmium surface. Many variants (all fairly acidic) have been developed subsequently all are based on dichromate (or chromic acid) with one or more of the following sulphuric acid, hydrochloric acid (or sodium chloride), nitric acid (or nitrate), phosphoric acid, formic acid and acetic acid. A survey by Biestek shows that several of these variants are as good as the Cronak process, although none is superior. 

Practical details of the Cronak process are given in Specification DEF-130, and a comprehensive account of the process as applied to zinc plate has been published by Clarke and Andrew. Fig. 15.5 shows the loss of zinc and the... 

The chromate film deposited by the Cronak process on zinc consists largely of a hydrated chromium chromate and contains some 10% by weight... 

Laboratory tests on plasters and cements are of limited value but can indicate trends. Lodder and Baumgarten (1951) looked at corrosion in damp gypsum plaster at room temperature. With only 0.046% chloride, corrosion was about 8 p,m/year in a 203-day test, even if the partly embedded rolled sheet was kept dry the steel was perforated in 52 days if kept damp. Chromating (Cronak process) virtually stopped attack (only 1.5 p,m/year). 

When 1.5% sodium chloride was added to the gypsum, significant pitting occurred at the interface on partially embedded samples that were kept moist chromating (Cronak process) virtually prevented attack in tests up to one year, but phosphating allowed corrosion to continue at the interface. 

Dipping. A process applied to treatment of ammunition and its component metal parts in an effort to protect prevent corrosion of the surfaces. Various chromate dippings (such as Cronak, Irridite, Yellow Black Anodize have been used especially when plating is first applied. The usual requirement of a dipping process is that the 24-hour salt spray test must be met Ref Ohart (1946), 14... 

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See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ , ]

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