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MATERIALS FOR PROCESSING APPARATUS 147
Precautions to be Taken when Selecting Construction Materials.
1. Do not permit tin, copper, or alloys containing these metals to come in contact with developing solutions, especially concentrated developers, because more or less of the tin or copper will dissolve and cause either serious chemical fog or rapid oxidation of the developer. Do not use galvanized iron vessels to mix developing solutions containing sodium bisulfite because sodium hydrosulfite will be formed, which is a bad fogging agent. Likewise, the zinc in the inner coating of galvanized piping will cause developer fog.
Contact of two or more different metals or alloys exposed to a developer will hasten the rate of corrosion of the metals and thus increase the amount of fog obtained. Soldered joints are particularly to be avoided with developers, but if such joints are unavoidable, a lowtin solder or one free from tin should be used, and the joints so made that a minimum of solder is exposed to the solution.
2. For fixing, toning, and acid oxidizing solutions, avoid metals whenever possible.
3. When choosing metal for the construction of apparatus, a single metal should be used whenever possible, and it should be either electro-welded or soldered from the outside to avoid electrolyic corrosion. Lead containers should be joined together by lead burning.
4. Apparatus constructed of aluminum, zinc, or galvanized iron should not be used with either developers or fixing baths since these metals react with such solutions with the formation of precipitates which leave a deposit on the film and often stain the gelatin.
5. Plated metals should be avoided whenever possible and only single metals or alloys used in preference, since electrolytic corrosion sets in as soon as a little of the plating wears off.
6. For fixing baths or strong saline solutions, avoid porous materials such as incompletely glazed earthenware, impregnated fibrous materials, or rubber compositions, because crystallization of the salts within the pores of the materials causes disintegration.
7. Tanks coated with lacquer or baked japan are not resistant to strongly alkaline developers or fixing baths of high acid concentration.
8. Avoid the use of cheap rubber tubing or other materials containing free sulfur or metallic sulfides in connection with developing solutions, because the alkali in the developer attacks these, forming alkaline sulfides which cause chemical fog.