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MATERIALS FOR PROCESSING APPARATUS 141
of the tank were soldered appeared to be slightly etched, and the original lustre of the metal had disappeared and was replaced by a dark, grainy deposit. The alloy itself was unaffected as far as could be detected from its physical appearance. A series of tests definitely proved that this excessive fog was a result of the tin constituent of the solder passing into solution, due to the flow of an electric current through the solder, the solution, and the alloy.
Corrosion was also observed due to the same cause when a tank made from this alloy and soldered on the inside was used as a container for an acid fixing bath, except that the alloy was corroded instead of the solder. When the joints were soldered on the outside, no developer fog was produced and corrosion was considerably less.
An extended study of this aspect of corrosion has been made and the results are given in two papers.*
Value of Various Metallic Materials
Only the practical application of the results of tests on the various metals will be considered in this article; the orginal papers should be consulted for more detailed information. Metals
Lead and nickel were the only metals tested which appeared to be of any especial importance for use with processing solutions although iron is of value for particular purposes. Lead, nickel, and iron (black or ungalvanized) tanks or piping can be satisfactorily used for most developing solutions although lead is attacked by strongly alkaline developers. Chemical lead is more resistant and is to be preferred to ordinary lead.** Tanks lined with lead or nickel can be used for fixing solutions but they are slowly attacked, become coated with silver, and must eventually be replaced. Plated Metals
Galvanized iron has long been used for the manufacture of washing tanks although it is not entirely suitable for this purpose. Vessels made of this material must not be used for mixing developers which contain sodium bisulfite, because the bisulfite attacks the zinc coating, forming sodium hydrosulfite which causes fog.***
Nickel plated brass is satisfactory for small developing tanks which are used intermittently. Metals plated with silver, either by deposition from an exhausted fixing bath, or by electroplating are more resistant to developing solutions according to the homogeneity of the silver coating, but their resistance towards fixing baths is only slightly greater than that of the unplated metals. Aluminum and cadmium coated metals do not satisfactorily resist photographic solutions. Chromium plated metals would probably be satisfactory if it were possible to secure a continuous non-porous coating over the base metal, but no such coatings are available to date. Lead coated iron can
* "The Effect hf Electrolysis on the Rate of Corrosion of Metals in Photographic Solutions" by J. I. Crabtree. H. A. Hartt. and G. E. Matthews. Ind. ft Fnq. Chcm. 16, (1924) 13, and "Cor'rosion of Monel Metal in Photographic Solutions" by J. I. Crabtree and G. E. Matthews. Ind. ft Chem. 16 (1924) 671.
** Obtainable from National Lead Company. Ill Broadway, New York, N. Y. Branches in all large cities.
*** "The Fogging Properties of Developing Solutions Stored in Contact with Various Metals and Alloys." by J. F. Ross and J. I. Crabtree, Amer. Phot. 23, (1929) 254.