Corrosion in Stainless Steel

Corrosion in stainless steel and especially puitting and crevice corrosion or not as widely appreciated in the yachting world as they might be, especially given the degree to which we all depend on the integrity of the many stainless steel components on our yachts.

This brief article is intended to explain how pitting and crevice corrosion in stainless steel arise and how it can be detected.

Stainless steel relies on an oxide layer for its corrosion resistance. This is called passivation.

Stainless steel contains a number of different metals one of which is chromium and this is largely responsible for the protection by creating a layer of chromium oxide or hydroxide. If this very thin layer is damaged the oxygen in the atmosphere can restore it.

Crevice corrosion and pitting corrosion occurs in stainless steel when this oxide layer is damaged, where there is a small scratch or crevice or pit and when there is insufficient oxygen present to restore the protective layer. Once that occurs and if exposed to salt water (chloride ions) corrosion begins and the initial crevice or pit is enlarged. Once the 'passivation' is brockjen down, this process becomes self sustaining.  

Below is a picture of three fastenings holding a gudgeon to a rudder skeg. These are effectively holding the rudder onto the boat. How do you think they look? OK? Need replacing? Perhpas need a little further investigation?

Well after tapping off the paint with my light hammer, I recommended they be removed asnd replaced.

This is what one of them looked like wehn it was removed. Note how the worst corrosion is where the nut was. Locking nuts with nylon inserts are not recommended for use with stainless steel because they are good at sealing out oxygenated water, thus peeding the process of crevice or pitting corrosion.

The products of crevice corrosion are a brown colour often seen on stainless steel fittings on yachts. It is very hard to predict how advanced any corrosion is because it is usually in a places that are not visible without dismantling things (eg the above bolt).

Here is a photo of a lower shroud chain U bolt on a Colvic Countess. Does it seem OK? There is brown staining but not very much. The rigging was replaced 13 months previously by a professional rigging conmpany. So might it be OK?

Here it is with the covering panel removed. The U bolt and the nuts need changing.

The threads of nuts and bolts are a prime place for crevice corrosion, especially when these are under load. Failure of nuts and bolts on yachts does not always follow if there is brown staining but it is prudent to assume that when brown staining is visible, the risk of failure is sufficiently high to warrant replacement of fastenings.

The quality of the tools that cut the threads in bolts and nuts is important as a poor quality of blunt tool can create crevices in new steel that will soon turn into a problem.

This crudely cut thread in an A2 stainless steel coach bolt has never been fitted to a boat. I simply boiled it in salty water for an hopur and left it in the open air for a year or so. You can clearly see how, despite it being exposed to oxygenated air, the damness, clorides and crevices have been sufficient to cause corrosion along the lines the cutting tool would have taken. Needless to say, these fastenings were not used for that job.