Corrosion caused by fasteners
When we talk about fasteners and corrosion, we are normally worried about the effect of the latter on the former, and how much the corrosion degrades the fastener’s performance, especially in terms of strength and particularly in terms of fatigue strength. Fastener corrosion is much more of a problem in other industries though, as many of the critical fasteners in a race engine operate in a pretty benign environment, especially those that are constantly bathed in oil.
However, there are conditions under which the fastener itself can cause significant damage to its adjacent components. In the presence of an electrically conductive liquid, for example, dissimilar metals in contact with each other can behave in such a way as to accelerate the corrosion of the most chemically reactive of them.
There is a ‘pecking order’ for the reactivity of metals, a property known as nobility. Metals that are very slow to react, such as gold, are termed noble metals, while those that are more prone to react are known as ignoble or corroding metals. Two metals with similar nobility, in contact with each other and an electrically conductive fluid, may not pose a problem – in fact, a problem may not occur with a pair of metals such as steel and aluminium. However, if the steel is substituted for stainless steel, or the aluminium is changed for a magnesium part, there may be serious corrosion of the less noble component.
The mechanism by which this happens is known as galvanic corrosion, and the nobility of each metal can be quantified by something known as electrode potential. Gold has the highest electrode potential, of +1.5 V, while the lowest electrode potential of any metal that we might use in the construction of an engine is magnesium, at -2.37 V. The material with the lower electrode potential of any pair will corrode, and the speed at which that happens is affected by the magnitude of the potential difference.
The potential difference in the case of gold and magnesium is 3.87 V, so it would be a very bad idea to gold-plate a steel fastener that would be in contact with magnesium in submerged or humid conditions. The electrode potential of the unplated steel fastener would be around -0.4 V, giving a potential difference with magnesium of -1.97 V; the gold plating increases the potential difference by a factor of almost two. Stainless steels are far more electropositive than non-stainless steels, and so can cause much faster corrosion of aluminium and magnesium than non-stainless steels.
Thankfully there are only a few instances where this is likely to affect a race engine, and these are generally restricted to fasteners in contact with the water cooling circuit. Some engines have fasteners passing through the coolant passages of the cylinder block, and the same can happen in the water pumps or elsewhere in the coolant circuit.
There is a balance to be struck – the engineer needs to protect against the corrosion of the fastener, but must also be wary of inadvertently accelerating the corrosion of the cylinder block or pump housing. Using deionised water rather than tap water can be an effective way to reduce the effect of galvanic corrosion while the water remains uncontaminated and hasn’t dissolved too many ‘nasties’. Also, corrosion inhibitors need to be used with caution, as too much inhibitor can make the coolant more electrically conductive and, perversely, accelerate corrosion.
Written by Wayne Ward