You have it: it’s the expectation that quality will be impacted once the diameter falls below the threshold. It can be done with careful application.

Steve Jones
Corrosion Management Consultant

www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

The appliers think in terms of square inches to cover, while the real issue is now many degrees of arc can I cover in each pass.
It takes almost as much time to properly cover 1.5" as it does 8".

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Thank you Steve and Ed. So, I am hearing two reasons.

a) Quality of TSA adhesion is lower due to inherent difficulty of TSA coating on curvatures of small diameters AND the use of flame spray instead of arc spray (read this difference in a TSA process in a handbook. Although they didn't speak explicitly about small diameters)

b) Applicators manhours are same...but they make less money on 1.5"?? I get the cost part, but does this mean, operators consider the choice of TSA cost prohibitive for small diameters?

The second part does make things more complex since small diameters have higher risk of CSCC on stainless steels. So, I would have thought it makes more sense to try and use TSA for smaller pipes (all else being equal).

Thanks again for your time.

Does your process require stainless steel piping? For small diameter piping there is quite a variety of corrosion resistant options, metallic and non-metallic.

What would lead you to think that external SCC risk is a function of diameter?
It is more expensive, relatively speaking to get good TSA and small diameter.
What other options do you have?

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Not necessarily quality of adhesion alone. Thickness control will be more difficult as an example. Inspection will also be more difficult. These are not insurmountable issues, and you will find some net pictures of coated piping arrangements with small diameters. A robust application procedure qualification, and careful thermal sprayer qualification raise the likelihood of a successful project. The added difficulties will add to the cost which then has to be weighed up against either the costs of a failure or the costs of a higher alloy material.

Steve Jones
Corrosion Management Consultant

www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

@TugboatEng
Yes, we needed SS and cannot change that.

@Ed
Came across that from talking to few experts in conferences and I also saw DNV-RP-G109 endorsing that opinion. It has something to do with difficulty of coating small diameters and shorter coating life. Debatable... but seems that concern extends to TSA and then the overall cost-benefit analysis might tilt either side depending on one's experience.
Options - I can always foil wrap it.

@Steve
Got it.
About a decade ago there was hardly any standard beyond DEP procedure for TSA quality control (unless I missed something). Now, we have two international standards detailing applicator qualification and requisite testing. It has served my purpose in the past. May be that's why I didn't see the issue surfacing.

Thank you all again. This helps.

SRPengineer

I do see a lot of foil wrap (under insulation) for SS pipe.
Two layers of heavy gage Al foil seems to be the way people usually go.
I know that wrapping is a pain, but then you do get the desired coverage.
In coastal plants you do see a lot of epoxy/urethane coated SS pipe and tube.

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P.E. Metallurgy, consulting work welcomed