Salt Spray Test & SS

[Paskenell]

We recently conducted some salt spray tests and are coming back with some data we find confusing. 5 stainless steel fasteners were sent to be tested by a well respected labratory in PA and 2 seperate tests were conducted on 5 samples of each of the 5 parts. We already had the material certs on the parts from the factory. They were a 1/4 Flatwasher, 1/4 X 3 Truss Phillips Machine Screw, 10 X 3/4 Flat phillips Sheet Metal Screw, 1/4 X 1 Hex Bolt, and a 1/4 Nylon Lock Nuts. All of these parts were purchased as 18-8 but had material specifications that fell within the 304 stainless steel requirements, except the washer which checked to 303. The first test, which all passed, was to ASTM-B117-97. We then asked the lab to run the parts continously until they showed rust. This test was performed to AMS QQ-P-35. The test was halted at our request at 500 hours. The bolt, nut and flatwasher showed no attack at 500 hours. However, the machine screw only went 24 hours till light red rust appeared and the sheet metal screw went 72 hours. I believe this could be due to the crevices of the phillips drive, which allowed a buildup of agents that successfully attacked the chromium oxide barrier. Or it might be due to the cold heading process for manufacturing screws, which is notorius for leaving microscopic fractures, allowing pitting. Are either of these two theories legitimate?

Confused?

 

[CoryPad]

Yes, they both may contribute. Were the parts from different suppliers, with different manufacturing processes? What about post-forming treatments? Pickling, passivation, etc?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Paskenell]

Thanks for responding Corypad. Yes the bolts, machine and sheet metal screws were made by one manufacturer (different plants), and the nuts and washers were from two others. The manufacturer of the first three and the nuts were passivated to ASTM A-967 and the washers were passivated to the old QQP-35C spec.

 

[CoryPad]

I would investigate variations in the forming (machining?) and passivation processes.

 

[mcguire]

Threads which are roll formed would have a big advantage over cut threads. Sulfur level would have a big effect. If crevices are present, they are not good, but I would bet on one of the first two I cited.

 

[Paskenell]

All four 304 parts measured below the .03% sulphur specification requirement, with the 303 washer falling below the .15% sulphur. The bolt, which went the full 500 hours, had the highest level of sulphur, though it fell within the requirements.

All threaded parts were roll thread and cold headed.

Thank you for the new angles to check.