A Negative View

The problem of corrosion caused by dissimilar metals being in contact with each other within a moist environment is a well known in engineering circles.

The problem occurs because in the presence of moisture, metallic materials in contact act as electrodes of an electrical cell and the material with the greater negative potential corrodes first. In addition, the higher the potential difference the greater the rate of corrosion. This has been given the name of electrolytic corrosion.

In aerospace design electrolytic corrosion is an important consideration during the initial scheming phase of a new project and the original design will limit the maximum potential difference between interfacing materials to – 0.25V for assemblies that will be exposed to atmosphere or salt water and – 0.5V for internal assemblies that are subject to condensation only.

In any engineering design, a component’s material is chosen for suitability to the task a particular component has to perform i.e. bronze for plain bushes.

However, the required material choice of constituent components making up an assembly may bring dissimilar metals into contact resulting in a maximum potential difference in excess of the recognised (aerospace) guidelines above and this is where plating is used to stop electrolytic corrosion affecting the integrity of the components.

Plating introduces a layer of material that will become the sacrificial element of the assembly at the interface of different metallic materials.

In the context of restoration it is possible, through a lack of knowledge, to adversely change the original design intent of an assembly and introduce a path for electrolytic corrosion that did not previously exist.

A simple example is in the use of home plating kits. Whilst these are a great innovation for the home restorer, caution should be observed.

As an illustration take steel brackets that were originally cadmium plated.

In the home environment it would appear ok to re- plate with nickel as a safer alternative to cadmium, which is cacogenic, producing a restored component with a visually pleasing finish / colour. With reference to the accompanying table, I’ll explain the error.

The table lists the most common materials found during restoration of a road vehicle and the corresponding potentials each one produces in water.

The cadmium plate has a potential of -0.80V and steel a potential of -0.75V. The difference in potential is -0.05V which being less than -0.25V is acceptable and as the cadmium has the greater negative potential in the presence of moisture it will corrode in preference to the steel base material of the bracket.

Now restore the bracket and finish with nickel plate applied with your home plating kit. The nickel plate has a potential of -0.15V and steel a potential of -0.75V. The difference in potential is -0.60V which being greater than -0.25V is unacceptable and adversely the steel now has the greater negative potential in the presence of moisture and it will corrode in preference to the nickel plate when the plating becomes damaged exposing the steel base material i.e. by tightening of fixing nuts on assembly.

Other ways to change the material interface, would for example, be the use of stainless steel fasteners (-0.20V or -0.45V) in lieu of original cadmium plated steel fasteners.

This can be acceptable but if the item being assembled was for example, made from bare aluminium (-0.80V), the change would adversely affect the material interface and introduce a path for electrolytic corrosion as the potential difference is greater than -0.25V and with the more negative potential the aluminium would corrode first.

I hope this article proves to be a useful guide into this subject for the MG restoration fraternity.

S. Cameron
TC0894 (under full restoration)

One thought on “A Negative View

  1. Tom Moore says:

    Interesting article highlighting the nature of galvanic corrosion, an often overlooked aspect of engineering design. While working as a materials engineer for a Canadian subsidiary of a multinational aircraft gas turbine manufacturer, it was common to call up a thin coating of silicone dielectric grease as a barrier between dissimilar metal flanges to ward off galvanic corrosion between cast magnesium alloy gear cases and nickel alloy or 410 (magnetic) SS gas generator and exhaust cases. The light weight of the magnesium was too alluring to resist, and when coated with 2 component epoxy paint and the silicone grease barrier coating the dissimilar metal joints performed acceptably in all but the most severe environments.

    I frequently smear silicone grease on auto engine joints and find the practice effective in warding off rust, particularly in “toy” cars used mostly on sunny days.

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