A case of Post-hydrotest corrosion and the comdey/tragic consequences associated with it!

Recently I received a query. Some SS321 seamless tubes have been hydrotested with DM water that contained 20 ppm chloride. This is a U Bend tube and a hydro test for a few seconds have been performed on the, later n by sponge passing through ID of the tube has been done and drying performed before dispatch. No chemical treatment done onthe water.

?As the corrosion engineer claims, “due to long storage outside we observed white circle with a pit at the centre”. Some seemingly phase analysis is also done and reportedly tunnelling effect in the steel is observed. The images sent to me were taken from a computer monitor and it was not evident to me which phase had been preferentially attacked. Apparently, the pits have been at 4 o clock and 8 o clock positions.

It is also to be noted thatafter the hydrotes, proper dry nitrogen filling is applied till it stored at site.

The query had three parts:

1.????? Is there anything in particular related to the tunnelling effect in SS321,

2.????? Could it be MIC?

3.????? What is the best biocide to apply?

First of all, how did you figure out that the case was MIC in the first place? just because you had a corrosion case so severe you could not explain?

?As a rule of thumb, one needs to understand that- as long as the temperatures are not high and they are within ambient temperature range- one has to first determine the “weight” of MIC and non-MIC? processes. In other words, we need to know if MIC and non-MIC are in series or parallel with respect to each other. Let’s remind you that in series corrosion processes, some corrosion reactions trigger other corrosion reactions (series) or can continue independent from each other (parallel).

In the particular case above, there are elements that ?can make us think twice. To the best of my knowledge, we don’t have a materials selection guide per material? as per susceptibility to MIC. Although I submitted a proposal for preparation of such a guide and standard and its urgent necessity to AMPP last year, such a standard is not yet written. So, although we can say that in general CS is more vulnerable to MIC? than stainless steels or that SS316/316L less susceptible to MIC than SS304, for a particular material it is really hard to say if it is susceptible or not. Then, you should report if you waited the hydrotest water between 24 to 48 hours prior to the test to let the DO decrease from ppm levels to ppb levels. You should also report features such the langelier and AI indeces of the water as well as its pH. Just because you are using DM water, it does not guaranteeprotecting your asset against corrosion, as a matter of fact if DM finds its way to become corrosive, it will be much more corrosive than seawater, for instance.? There are four post-hydrotest corrosion scenarios only one of which being MIC.? So don’t be quick to jump on the conclusion that MIC was the main culprit and then look for the best biocide to treat it!!!!

So, don’t be quick in jumping to the conclusion that the case of corrosion is MIC. Last but not least, with due ?respect to all, do not expect" free landing" either: you have a problem and you must be prepared to pay for it.

When it comes to MIC, most of the time the inquirer comes along passionately with too much information that can have no relationship to characterisation of the case because they are, most of the time, all collected not based on recommendations by the expert but by a professional who may not necessarily be an corrosion expert or an MIC expert.

You respond to the question asked of you but then, if they ever do, they will come back to you with one of the below formats:

1.Our experts solved it, thank you!(where were your experts when you realised the problem then?)

2.We are still waiting for the management to approve expenses, (oh yeah? “Tomorrow never comes”!)

3. We cancelled the job! (1. they are lying or 2. Instead of finding a solution, they preferred to erase the question!)

A physician friend of mine says when she attends some gatherings even a birthday party, as soon as they understand she has a medical degree, they keep asking her questions about their real or imaginary illnesses and ask for the best treatment. She says my usual response to them is always the same: come to my office, be prepared to pay for your visit and the tests I write for you so that I can put a symptom on your illness and choose the best treatment for you” When I asked her about what happens next, she said: a good fraction of them do as I say, a minority go for the so-called “alternative medicines” and say behind my back that I see nothing but money!!”

To me, what she said was so familiar, unfortunately.