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.

The drain mystery

I recently moved to a new house. As expected, there was a lot of cleaning up to do. One of the tasks was the cleaning of the washbasin. Usually, the drain is a circular part with  5 to 6 holes for the water to flow out. What I saw was this –

I have not had a chance to analyse the material of the drain. However, a quick search tells me that this is most probably stainless steel. The water that this drain is exposed to is the bore water. Thus, the drain has encountered a lot of chlorides. There is general as well as localized corrosion.

The damage started off as a simple process of pitting. Pitting due to chlorides is one of the most common headaches for poor stainless steel. They break the passive oxide film, and reach the underlying fresh iron. This iron then reacts with the usual suspects (ions, oxygen, water) and forms what we see as the rust.

As can be seen in the picture, the thin sections of the drain between the holes have disappeared in three places. This may have happened because the pits formed continued to grow through the thickness of the material, which finally gave way and fell down the drain. The shape of the circle to the top left is distorted and we can observe a small nick in the circle. There is also a variation in the widths of each of the sections between the circles – all because of corrosion.

Then, there is the green color. This is the corrosion product ferric chloride. A quick look shows the green formation around the part between two holes at the top right. General corrosion is visible, and there may be pitting going on underneath the corrosion products.

We will have to wait and see if that small section is the next to break off.

UPDATE: It did break off.

Corrosion – the bane of all the industries

Corrosion is the journey of an element from its most active form to its most stable form. 

The birth

At the place of its birth, inside the earth, an element such as iron comes into contact with atmospheric oxygen, chlorides, sulphate, and moisture. and is quite happy to remain in peace with them in the form of Fe2O3, Fe3O4, FeCl3 and such.

The growth

It is then extracted and made to go through a number of processes which separate it from the ions and bring it to its elemental form. It is then put into use to make steel, where it coexists with carbon, manganese, chromium, and other alloying elements.

The interaction

However, iron has three electrons that it desperately need to give away. That is exactly what it does the moment the steel is brought out from the factory and put outside in the shed. Iron is more than happy to reunite with its old buddies – oxygen and moisture. It gladly gives away its electrons to them and returns to its peaceful state of Fe2Oand Fe3O4.

And that is how the corrosion of steel begins.

😀Happy learning!😀