Corrosion rate and pipe design

Corrosion is a serious issue. However, corrosion engineers are rarely asked about it during the designing of components. It is only when the components fail that people remember there are people who have studied corrosion for their whole life and would provide a solution. The design problems are really quite simple, and the loss of money could have been avoided had the company bothered to involve a corrosion engineer in the first place.

Let me illustrate this with an example of a factory near the sea.

Suppose a mild steel pipe is fitted inside the factory to transport 1 wt. % hydrochloric acid. The engineer has a choice between selecting pipes of 5 mm and 10 mm thickness. To save money, they go for the pipe with the 5 mm thickness because it has been 'successfully used by the other customers'. Over the course of a year, it is seen that the pipe has started leaking at certain places. Further investigation reveals that those sections have thinned to half of their thickness.

Now, a coupon test is carried out to assess the corrosion behaviour. The corrosion rate is calculated to be 2.5 mmpy, which means the alloy was prone to corrode and lose 2.5 mm of its thickness in one year. Hence, the corrosion rate in the design of this pipe should have been crucial in the beginning itself, because now the company has to bear the cost of not just the original pipe, but also the new pipe with greater thickness.

So, the company decides to go for a 10 mm thick pipe for the transportation in the enclosure.

It further decides that it will construct a piping system that enables the loading of the acid from outside the enclosure. Hence, it extends the pipe to the outside of the factory.

Sure enough, they are back to square one within a year, wondering what went wrong with the pipe outside the factory.

The answer was in the extra plans that they made. The recommendation of 10 mm diameter pipe was made for the pipe in the internal environment. The problem was in the extension of the pipe to the exterior.

The exterior of the pipe was exposed to the moisture and ions in the air. The corrosion rate of the steel in air is 2.5 mmpy. So now the pipe was undergoing an internal as well as an external corrosion. As a result, the total corrosion it was experiencing was once again 5 mmpy.

This led to the corrosion of the pipe on the exterior.