What type of Corrosion 1

[nabeel3]

Greetings
What type of corrosion is this? Pitting or General corrosion or
Carbon steel Deminilarised Insulated Water Tank. This is found after removing insulation. tank
Tank is near to sea .

 

[mfgenggear]

OP
what is the tank material?
when was it installed?

 

[nabeel3]

Tank is Carbon steel. ASTM A 283-C or A 573-70. Built around 2016

 

[mfgenggear]

OP
the tank appears to have a galvanized coating, which may be not thick enough or had incomplete coating. moisture will penetration through the plate.
causing corrosion. depending on the type of
insulation, the chemicals in the insulation may attacked the coating and the steel.

 

[mfgenggear]

op
did the insulation have barrier on both sides.
or was it on the outside only allowing contact with the insulation.

 

[nabeel3]

This is all what the client provided. Only outside barrier for insulation

 

[SJones]

If it was insulated with the normal materials, then the corrosion is more likely to be classed as corrosion under insulation CUI. Next time ensure a top quality, qualified tank lining coating, or dedicated anti-CUI coating, and a top quality insulation job.

S-738 - Insulation for Piping & Equipment

The purpose of this specification is to define a minimum common set of requirements for the procurement of insulation for piping and equipment in accordance with NORSOK M-004, 1st Edition, December 2018, Piping and equipment insulation for application in the petroleum and natural gas industries...

www.iogp.org

 

[nabeel3]

is this pitting

 

[LittleInch]

I would describe this as general corrosion with some areas consisting of multiple deep pits and some isolated pitting.

"pitting corrosion" is more normally like acne - multiple small pits of variable density and depth. once they grow into each other like it ooks here, then it becomes a larger general corrosion with some variation in depth.

why do you want to split hairs?

 

[hacksaw]

Salt water corrosion

 

[mfgenggear]

We did not get a picture of the insulation so we have to use educated guess. But with the correct cover and insulation .it should have
Protected the surface. If the top is not covered. It defeats the purpose. As noted is appears liquid was running down from the top. Evident by the first photo.

 

[EdStainless]

Looks like it was galvanized or coated, and the surface was damaged in some locations.
Either your weather proofing and drainage is poor or you have a lot of condensation.
Are you sure that he insulation had no leachable chlorides?
You will only know about the pitting after you gently grind some of these and measure how deep it is.
If thickness is acceptable then you could remediate the tank.
This would be similar to what is done for water towers.
Clean the surface back to white metal and use a three layer coating system with a zinc-rich primer as the base layer.

 

[weldstan]

Was there ever a sacrificial anode installed? And agree with EdStainless re remediation.

 

[mfgenggear]

There is no remediation, the corrosion has to be cut out and replaced. Once corrosion starts it has be completely removed.

 

[EdStainless]

This tank was designed with a CA.
If they can grind back and still meet the required wall then they can remediate it.
A good grind, blast, and clean topped with a Zn rich primer (that is then top coated to protect it) will work fine.
These are often three layer systems.
And location that is found under wall will need to be cut out and replaced.

 

[mfgenggear]

Ed stainless that's the issue . The unknown depth of the corrosion. One may grind off all the visible corrosion, how ever there may be
Intergranul attack which is not visible to the eye. Then there is an mrb engineering disposition if the remaining wall is acceptable. For the stress strain factor.
I would as my opinion I would rather cut it out and patch it.

 

[EdStainless]

Engineering judgement.
I know of a company that regularly requires replacement of such section in the lowest course of a tank, but allows remediation above that level.
If I have a concern about this tank it is that it is fairly new.
I would expect this with 25 years of service, but only 8 years?
Something is wrong.

 

[mfgenggear]

Agreed ed