Dr. Bob Q&A about Corrosion
Dr. Mroczkowski, my question is this…
In your recent articles in ConnectorSupplier.com (and in similar papers from your AMP days) you have basically stated that the reason interface resistance is lost when corrosion enters the picture is because corrosion films and contaminants compromise the asperity interfaces. However, I’ve never been able to quite picture what this actually involves. In other words, what is physically/mechanically happening to bring about this “compromise?” Does surface corrosion creep toward an asperity and then “worm” its way in-between the two surfaces of the asperity from the outside perimeter? If so, wouldn’t that tend to push apart the surfaces of the asperity as soon as corrosion worked its way just inside the outer perimeter of the asperity, and lead to an almost immediate loss of the entire asperity? And, if those surfaces are moving apart, wouldn’t this result in nearby asperities being quickly and totally compromised even if there were no corrosion at those asperities? Or is the degradation mechanism something entirely different? I would appreciate it if you could more fully explain or illustrate what is actually happening during the degradation process.
Dr. Bob says: There are two answers to your question, one theoretical and one “realistic.” Theoretically, corrosion can occur at each individual asperity contact point. Remember that the asperities are distributed over an apparent contact area that is determined by the geometry of the contact springs at the interface. Assume a circular area, a sphere against a flat as an example. There will be asperities distributed throughout the apparent contact area. The corrosive environment will react with the outer asperities first, and later diffuse into the inner asperities as well. Over time, corrosion could first reduce, and then eliminate, all of the contact areas and an open circuit would result. Naturally the resistance would increase incrementally, but at an increasing rate as corrosion reduced both the distributional area, as well as that of the individual asperities. Modeling of this process indicates that the time span for this to happen is of the order of decades, so this is not the observed degradation mechanism in the field.
Your corrosion-related “push apart” mechanism is generally not observed, though there are special cases where it can take place.
The “realistic” mechanism is that the corrosion areas build up around the asperity contacts and some sort of micromotion event, as described in the fretting corrosion article, causes the asperity contact points to move. Some of them will move onto the corrosion fields surrounding them, resulting in an incremental increase in contact resistance, as individual asperity contacts are lost initially, and at an increasing rate, as the distributional area is also decreased over time.
This mechanism could be described as a general case of fretting corrosion because the degradation is driven by the fretting motion that displaces asperity contacts onto the corrosion areas surrounding them. In that sense it is very similar to conventional fretting corrosion as experienced in tin-to-tin contact interfaces. The difference between the two cases is that in tin-based fretting corrosion it is the contact finish, the tin, that is susceptible to corrosion. In the case of gold finishes, the corrosion source is the exposed base metal, not the gold finish itself.
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