HIC/SOHIC resistant coating 2

[Corgas]

In order to protect bottom of a new pressure vessel in Wet H2S, besides SS cladding or Weldoverlay, we're considering use of a baked phenolic type coating to protect it from HIC/SOHIC/blistering.

PV of A-516 Gr 70 (PWHT). Service meets criteria of sour service according to NACE MR0175.

Any experiences with these types of coatings?

 

[rustbuster]

There are many good coatings for this application and they will help prevent these types of damage. Talk to a supplier / applicator.

Good luck

 

[SJones]

Coatings will NOT prevent HIC in a susceptible material although they may delay its onset. Coatings will degrade in service and allow the H2S access to the steel surface at some point. The key word in the foregoing is 'susceptible': specify the steel with good chemistry control and verify its performance with a HIC test and the 'susceptibility' factor is removed from the equation. If you are looking to control an expected metal loss corrosion (not cracking) problem, then coatings may offer a satisfactory solution based on the criticality of the vessel and life cycle costing accounting for the probable need for more frequent vessel inspection and in situ coating repair when compared to a correctly selected CRA.

If you deduce that a coating offers the best corrosion control option, then make sure that you select a coating based on appropriate test data which should include a decompression resistance assessment. NACE Corrosion 2004, Paper 04034 Internal Vessel Lining Strategy For A Major Oil Company would make a good start in your bedtime reading list.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Corgas]

Thanks Steve!!.

What you wrote I have read it also in NACE publication 8X194 Appex C. Quote : "Coatings have been found to be permeable and have tended to blister as a result of hydrogen and other light gas diffusion." But in same appex C in the introduction : "Atomic hydrogen is generated by the corrosion reaction; therefore, if corrosion is stopped, hydrogen cracking or blistering cannot occur".

If coatings isolate or prevent contact of Steel (Fe) with H2S, how can there be still a corrosion reaction?

Same appendix, NACE endorses Baked phenolics : "Baked phenolics have demostrated success as protective coatings for continuous wet H2S refinery service conditions below 250 °F".

We do have an in-house spec for HIC resistant carbon steel plates : Low S & P, CEq, testing accord to TM0284,etc.

I have another question for you :
is it worth it to spec out HIC resistant steel if SS cladding or Weldoverlay is going to be applied?

Finally, have you had experience in monitoring Hydrogen permeation?

Corgas

 

[SJones]

That is the point that I'm trying to make: the coating may be 'perfect' on Day 1 all holiday tested and hunky dory; but, what's it like on Day 10, 100, 1000? Coatings will break down at some point and allow access of the fluid to the carbon steel where the corrosion reaction can take place. What happens to the coating when the inspection crew enter the vessel with all their equipment banging around? What happens if you have to blow the vessel down quickly from 100 bar and the permeated gas can't escape from the coating quickly enough and blows it off? It's a question of the risk culture that your company has when, on the probability side of the corrosion risk equation, it's known that coatings can be problematic.

As to the use of fully H2S resistant substrate steel for cladding, again it's down to perceived risk. How good is the cladding process? How reliable is the NDT of the vessel fabrication and overlay? Will your in service inspection find corrosion before it's too late? I used to work on an increased steel cost of 7% for HIC tested steel so is 7% too much versus getting risk to as low as reasonably practicable? On the other hand, HIC will only lead to a leak and not a rupture and modern steels are pretty clean anyway (the 7% uplift in price is more to cover the remake cost risk of a HIC test failure rather than the cost of different steelmaking!). So, do you take a flyer on not getting HIC in a non-HIC tested steel if it gets exposed at a fault in the cladding? Could be a reasonable bet because HIC will not be rapid, should be readily detectable from outside the vessel and any exposed area of steel substrate will be relatively small giving a low hydrogen flux which brings us on to hydrogen probes. Never used them because of what to do with the data. What does a hydrogen flux of X really mean for the material. Sure, it tells you that there is a hydrogen generating reaction going on but what does that mean for metal loss or cracking that a good UT inspection couldn't tell you? All it can do is give you information to assess the corrosion risk which takes you around the loop again.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[gr2vessels]

Corgas, you have to read the ASME SA 264 for details of the cladded plate for pressure vessels. Yes, the carbon steel must be selected, tested and certified for compliance with NACE MR 0175, otherwise any small crack in the overlay will lead to an accelerated pinhole corrosion and rapid failure of the vessel.
Also, the welding procedures have to be compliant with the NACE requirements. It's not a big deal the whole compliance story, just tests and records for certification. Depending on the risk of failure of the cladding layer, you might use an additional coat of some paint or other covering, but the benefit of it is really questionable.
The degree of corrosion protection depends on the quality of the cladding material used, starting with the basic Austenitis Stainless Steel up to any exotic CRA material. It has to be selected as per NACE requirements (and your extended tests if required).
The fabrication methods are critically important, in order to make a good use of the materials and to prevent ruining them with bad welds, PWHT and shortcuts in manufacture. The paint might extend the 25 years of service life by 37.3 days, so much for the paint expenses. In other cases, the paint is all you need.
There are many carbon steel vessels simply coated with suitable H2S resistant paint. The coating break down can be monitored due to electric conductivity of the fluid in contact with the carbon steel shell. Then you stop the process, re-coat the interior and start the process again, monitoring the coating break down.
Cheers,
gr2vessels