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February 1971

By: J. D. Morrison

Abstract

This report covers the work accomplished from February 1968, to date on a program, conducted for the Mechanical Design Division by the Materials Testing Branch, to evaluate the performance of austenitic stainless steel alloys used in fluid systems lines at KSC. Need for the program was dictated by the occurrence of numerous failures of stainless steel hardware, caused by pitting and stress-corrosion cracking, over the past several years. Test have been conducted to determine the inherent corrosion susceptibility of several alloys - AISI Types 304, 304L, 316, 316L, 321, and 347 - and to evaluate the effectiveness of certain sacrificial-type protective coatings in preventing corrosion failures. the test samples, both unprotected and coated, have primarily been tubing sections and tubing assemblies employing 37deg -flare fittings. Samples were placed in racks approximately 100 yards above high-tide line at Cape Kennedy. The racks were designed to provide complete exposure of half of each tubing sample and shelter from direct rain impingements of the other half. Protective coatings and treatments evaluated include organic- and inorganic- zinc-rich paints, an aluminum filled proprietary coating, and periodic surface treatment with a phosphoric acid wash.

General conclusions reached at this point in the program are as follows:

• All of the unprotected tubing samples, regardless of alloy type, showed evidence of pitting initiation after about two-weeks exposure at the beach test site.
• Samples of Types 321 and 347 appear to develop a larger pit population than the other alloys.
• The deepest pit penetration (about 65% of the wall thickness) that has been discovered in the bare samples examined to date has occurred in Type 316 tubing. However, it is probable that actual pitting rate is independent of alloy type and that no one of the alloys evaluated has appreciably better resistance to pit penetration than the others.
• The deepest pitting generally occurred in the sheltered portion of the tubing samples, probably because of the retention of deposits from salt fogs.
• Zinc-rich coatings, both inorganic-base and organic-base, and an aluminum filled coating, have afforded sacrificial protection to the stainless steels against pitting, for as long as 28 months, and against stress-corrosion cracking of fittings, for as long as 12 months. It is believed that a much longer effective coating life can be expected.

For additional information, a complete copy of this study is available as NASA Report KSC-MAB-431-68.   Send requests for copies to corrosion@ksc.nasa.gov.

To download a full copy of this report in Adobe's pdf format for local printing, click 431-68.pdf.  (2.2MB)

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