Corrosion - Mixing your Metals

When corrosion is found electrolysis is often blamed as the prime suspect. However, this is not always the case. Corrosion can and does occur without any stray currents being involved at all. Understanding how and where corrosion can occur will help to prevent or at least minimise what can be costly damage over time.

Basically there are two types of corrosion: corrosion caused by mechanical friction like sandy water or high speed water flow, and corrosion caused by electro-chemical change, which includes electrolysis. This is called Galvanic Corrosion and is more prevalent when water, particularly salt water, is present.

Where friction corrosion is concerned, the ability of quality metals to withstand such impacts is obvious. Understanding the process of electro-chemical corrosion is rather more complicated.

Metals contain thermodynamic energy - positive and negative charges - which is freed by the process of corrosion, and are rated in their ability to resist electro-chemical corrosion on the Scale of Nobility on the Galvanic Series chart.

The more noble, less active metals have a neutral or negative electrical potential, so will not generate a flow of positive ions. And less noble metals are more active with high positive charges which do generate electrical current flow.

Gold, platinum and palladium are among the most noble and are generally safe from being corroded, whereas Zinc, unalloyed aluminium, iron and steel are among the least noble, with graphite and carbon being most highly charged. Bare steels (like 4130) are very active. When exposed just to normal atmospheric moisture and oxygen, a layer of red rust can build up through oxidation - and with each rise in the temperature of 10 degrees the rate of oxidation is doubled.

Of course not all metals are pure. There is a very wide range of alloys. Alloys mix metals and these mixtures can be a recipe for corrosion. For example, all bronzes, brasses and stainless steels, are not the same. So it is important to know how noble a particular alloy is. Moreover, not all metals look like metals. Rubber does not have a metallic appearance, but it contains carbon, and carbon rubber mated to stainless steel can produce quite a reaction!

Electro-chemical corrosion has been given a number of names derived from the circumstances in which it is found. The most common forms you see are galvanic, crevice, pitting, uniform and stress corrosion, as well as electrolysis.

Galvanism occurs when two metals with different electrical potential are in close contact, creating a current flow. In dry conditions, the resultant flow is very small but when the metals are joined by water, the water acts as an electrolyte, and there is significant in electron flow from the more active metal to the less active metal, resulting in particles of metal being deposited from the less noble to the more noble metal.

This is very prevalent at sea due to the high conductivity of salt sea water, which in effect forms a natural battery. Oxidation occurs at the more active (less noble) metal, known as the anode, creating a flow of electrons to the less active (more noble) metal called the cathode.

The further the metals are apart on the Scale of Nobility or Galvanic Series chart, the greater the potential for electro-chemical corrosion to occur and the extent. Commonly you can find aluminium being transferred to more noble copper as the result of electro-chemical corrosion. The potential is also greater where the more active metal is small and the less active metal has a large surface area. An aluminium rivet in a stainless steel tube connected by water will corrode very quickly indeed.

To avoid problems it is wise to use metals that are as close as possible to each other on the galvanic scale. Either the same or a more noble metal should be used for small fasteners and bolts. If you do have to joint dissimilar metals, you will need to ensure that the less noble metal has the greatest exposed area, and mating surfaces are insulated either by jointing compounds or properly designed inserts and sleeves.