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"Is rust contagious?"

October 23, 2017

Q. Sorry for the salacious posting title.
I have a threaded assembly with a steel washer and a brass plated housing. The steel washer has rusted out and the threaded housing has rusted through right next to the washer but has no signs of rust anywhere else on the housing. So my question is, "Does the rusting of one part, cause rust on the other?"
I ask because I am hoping to fix the problem by simply changing the steel washer to a SS one to prevent the first part from rusting and thus protect the threaded housing.

Charlie Harland
- Kitchener, Canada

October 2017

A. Hi Charlie. Rust is not contagious. Rather, things that are very close together tend to rust similarly because the same environmental conditions prevail. Water probably accumulates in the gap between the threads and the washer. Put some thread sealant, or at least teflon tape, on the threads to discourage moisture accumulation and the situation should be somewhat improved.


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

January 23, 2018

A. Once one area of the surface has become oxidized form of the metal, now this corroded portion of the metal will exist at lower potential relative to the bare metal. This variation in surface potential will drive galvanic corrosion in same mechanism that is caused by dissimilar metals. In effect, this can also be used to explain pitting. The pit is active metal and the perimeter is the protected oxide. Therefore, once corrosion products form non-uniformly across the surface, then you are in trouble and should try to remove it or cover it. Passivation of metals before application can form a homogeneous oxide layer and this layer not only becomes barrier, but it limits variations in surface potentials as well as conductivity thru electrolyte interface. In summary, corrosion/rust could be considered contagious because of the way it can spread.

William Rigdon
Stanley Black & Decker - Baltimore, Maryland

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