Effects on Structural Aluminum Alloys

HEDRICK, H. G. (General Dynamics/Fort Worth, Fort Worth, Tex.), C. E. MILLER, J. E. HALKIAS, AND J. F. HILDEBRAND. Selection of a microbiological corrosion system for studying effects on structural aluminum alloys. Appl. Microbiol. 12:197200. 1964.Two laboratory methods, a metal-strip test and a tank test, were evaluated as microbiological corrosion systems for producing corroded test specimens on a structural aluminum alloy. The results show that corrosion of the test alloy occurred best in the metal-strip test in a deionized water-fuel medium inoculated with a mixture of microorganisms under aerated conditions. The metal-strip test was more successful for producing large numbers of corroded test specimens and proved more economical than the tank-type test, since less structural material is needed to obtain a specimen with sufficient corrosion areas, and since the corrosion can more easily be restricted by maskants to certain areas for specific test purposes. Microorganism contamination of integral fuel tanks in jet fuel aircraft has been recognized as a serious problem in the past few years. Mod-Maintenance programs have revealed large deposits of slime or bacterial growth on the bottom and sides of fuel tanks. Widespread corrosion has been detected beneath these deposits in varying degrees, from small isolated pits and areas of exfoliation to extensive corroded effects. To study the effects of microbiological corrosion on structural materials used in aircraft fuel tanks, it is desirable to produce such attack on a large number of specimens under laboratory conditions. Considerable information is available (Harris, 1962; Baumgartner, 1962; Rogers, 1948; Starkey, 1956; Baudon, 1958) on microbial corrosion of metals other than aluminum. Recent reports (Churchill, 1963; Ward, 1963) have attributed the corrosion of integral fuel tanks to the presence of microbiological contamination. However, little, if any, information is available on the methods for producing microbiological corrosion on structural aluminum alloys under laboratory conditions. The purpose of this study was to select a microbiological corrosion system capable of producing corrosion in the laboratory which duplicates that found on naturally corroded integral fuel tank specimens. MATERIALS AND METHODS In developing the microbiological system for determining the best system to produce corrosion under laboratory conditions, two approaches were used. One utilized metalstrip type tests and the second employed tank type tests. Metal-strip test. The materials for the metal-strip type test included cultures of Pseudomonas aeruginosa GD/FW B-3, Desulfovibrio desulfuricans Prince, ASD, Cladosporium resinae QMC 7998, and Aspergillus niger GD/FW F-1, which were used singly and in a mixture, and a natural microflora from a contaminated fuel sample collected at Ramey Air Force Base, Puerto Rico. This sample contained primarily Pseudomonas type microorganisms. The inocula were prepared from laboratory stock cultures growing in a Bushnell and Haas (1941) fuel medium for at least 7 days. A 1-ml portion of each inoculum was added to each test jar. The inoculum used in the deionized water test setups was prepared in tubes of deionized water-fuel before use. Each inoculum was checked for viability by the streak plate method. The test media used were (i) Bushnell-Haas fuel, (ii) deionized water-fuel, (iii) Bushnell-Haas-cystine (50 mg per liter)-fuel, and (iv) seawater medium-fuel (Sisler, 1961). All of the media were used at a 2:1 medium-fuel ratio. Test specimens (1 by 4 by 0.125 in.) were prepared from aluminum alloy 7178T651. All specimens were coded, cleaned, weighed, and then sterilized with the media in 16-oz culture jars at 121 C for 15 min. The position of the specimens in the jars was FIG. 1 Metal-strsp test setup for screening the microbiological corrosion systems. 197 on N ovem er 2, 2017 by gest ht://aem .sm .rg/ D ow nladed fom