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From: Azri Syahmi
To: Controls Manufacturing Community List
Subject: Stainless steel tubing

Happy new year to all.

This is my first question for this New Year. We normally specify 316SS tubings. When the tubing arrives, it will come with all the necessary certificates confirming the material compositions etc. But once it is installed, the tubing starts to show signs of corrosion typically pitting corrosion.

The environment is seaside and theoretically, 316SS shall not corrode in this environment. I cannot rely on the certificate alone. Appreciate feedback on how I can detect substandard materials.

Best regards

Azri Syahmi

From: Ian H. Gibson
To: Controls Manufacturing Community List
Subject: Stainless steel tubing

Bear in mind that 316 stainless is not resistant to chloride corrosion at temperatures above about 65C - solar exposure in low latitudes can easily exceed this. Salt spray exposure is going to give pitting. Even duplex stainless is not proof against hot, wet chloride solution.

Ian H. Gibson CPChem RPEQ FSEng
Process, Control and Safety Engineering Consultant
Pascoe Vale, Vic., AUSTRALIA

From: Luiz Antônio Dourado Gonçalves
To: Controls Manufacturing Community List
Subject: RE: Stainless steel tubing

Azri,

I faced similar problems in the plant I work. We are close to the sea, in a petrochemical complex and with problems in tubing and instruments external cases (like the external body of secondary compartment of mass flow meters). Talking to two materials experts, both mentioned the same thing Mr. Gibson says and also recommended me to paint all 316SS exposed materials, including OD type fittings. This will avoid pit corrosion and, in the long run, avoid need for replacement. At first it seemed weird to me, now, after five years I admit they were right.

Luiz A. Dourado
Salvador - Bahia - Brazil

From: Azri Syahmi
To: Controls Manufacturing Community List
Subject: Stainless steel tubing

Luiz,

Thank you for your reply. Painting helps to protect the external surface. Do you face any problem of corrosion inside the tubing?

Regards,

Azri Syahmi

From: Walter Driedger
To: Controls Manufacturing Community List
Subject: Stainless steel tubing

316 SS reacts to chlorides, i.e. sea salt. If you have salt water inside the tubing, you will have problems. In fact you will have problems with chlorinated water of any kind inside SS tubing.

I was somewhat shocked when I first started to learn more about 316 SS. It resists rusting from air but that's about its only really good feature. It is not especially strong nor hard and it has a bad habit of galling for the same reasons as aluminum does.

The point about painting SS - I expect the paint will peal off in short order. Does Luiz have experience with this?

I think copper tubing is good against sea air but I don't have any personal experience. One thing for sure, the worst is aluminum. On the other hand, all copper alloys, including brass, are extremely susceptible to H2S corrosion while Al is very resistant. If I had a sour gas plant by the sea side I wouldn't know what to do.

Walter Driedger