corrosion in cast iron = graphitisation

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Cast iron differs from steel in that it has a higher carbon (>2%) and silicon (> 1%) content which makes it cheaper to produce than steel. Cast iron corrosion resistance is comparable to that of carbon steel or even better depending on the alloy content. In view of the high carbon content, graphite, which is cathodic compared with iron, forms in the alloy microstructure where it can be found as chips (grey iron) or nodules (ductile iron) depending on its composition and heat treatment.

This potential differential creates a mechanism known as graphitic corrosion or graphitization; this phenomenon occurs in non-alloy cast iron exposed to medium acid or soft water. Water that has a low hydrogen sulphide content (1 ppm) also encourages graphitisation. Graphitisation tends to penetrate into the metal but at slow rates of progress. When this type of corrosion occurs, a layer of iron oxide containing graphite forms on the surface. This layer preserves the shape of the part while corrosion is taking place and, therefore, this type of corrosion cannot be detected by means of a visual inspection.

The service life of most cast iron components in aerated and alkaline domestic water systems at ambient temperatures has proved quite satisfactory, mainly due to the thickness of moulded parts and to a uniform, moderate level of corrosion.

In the distribution system, cast iron network longevity can exceed one hundred years but, in the main, these systems will require protection in the long term. In days gone by, the inside of cast iron pipes used to be protected using simple linings of the bituminous varnish type. Nowadays, we use cement mortar based linings that will also withstand wastewater that contains sulphides. We are witnessing an increasing use of linings based on an approved plastic material.

External protection against corrosion often consists of a several hundred micron thick bituminous coating. Added protection can be provided at coating joins in the form of sacrificial anodes, zinc spray or a cathodic protection based on the potential imposed by the corrosive nature of the ground.

The use of tubes constructed of ductile iron has increased strongly due to their high mechanical strength. These tubes have a slightly shorter life expectancy compared with those constructed of grey iron because they are thinner and because of a faster rate of initial corrosion.