Crevice corrosion 1

[CWicker]

We are using backing rings in 304L piping where it is difficult to obtain a satisfactory purge. The service fluid is borated water at 25 deg Centigrade. The piping has proven to be extremely susceptible to SCC from external chloride contamination and high residual tensile stresses. Therefore, with the high residual tensile stresses present in the pipe and at the welds, is crevice corrosion in this internal invironment (2000ppm borated water) a real risk? ASME approves the use of backing rings, but theory would suggest that crevice corrosion can occur.

 

[Metalguy]

At 25 deg C you won't have to worry about ID SCC, crevice or not, with pure water plus boric acid (probably sodium borate, right?).

 

[CWicker]

We dose the system with crystalline boric acid - it is the nuclear safeguard system of a Pressurised Water Reactor. The 304L has suffered severe transgranular SCC deradation on the external surface and our concern is that it will be especially susceptible to any form of impurity or oxygen depletion within a crevice.

 

[Metalguy]

Cl related SCC in non-sensitized 304/316/reg/L is almost always transgranular-not that the SCC mode makes much difference.

While crevices are usually to be avoided, low temp. and boric acid should not be a concern IF the halogens are very low. But remember, crevices act as concentrators for them (halogens), so it all depends of the amount you have. But the boric acid is a very weak one, and does not cause problems by itself in SS especially at such low temps.

Can't you replace the OD-damaged areas? Maybe you can weld a sleeve around it, assuming it's not a long length of pipe involved.

 

[CWicker]

Thanks for the responses Metalguy. We have performed an extensive refurbishment programme on about 4000m of 304L piping - we surface buffed ALL the pipe and inspected with liquid dye penetrant. ALL defects (pits, cracks, scratches etc) were then removed by deeper buffing (while respecting minimum wall thickness requirements). ALL leaking pipes and any with SCC deeper than min wall were replaced (about 300m). Hence you can see our sensitivity to any mechanical arrangement that may induce SCC in piping that is thought to be essentially resistant to SCC at less than 30 deg C.

 

[mcguire]

CWicker
I would have agreed with Metalguy that 304L shouldn't SCC at 25C. There are plenty of data and opinions out there that suggest that the problem only comes at higher temperatures. I'm beginning to believe that you can have SCC in austenitic stainless whenever you have pitting.
I do suggest that the remedial mechanical buffing of the surface may be mostly estheticly pleasing and may not remove the initation sites for SCC. There's good evidence that initation sites are inclusions of manganese sulfide which are surrouded by unstable austenite which not only corrodes more readily that the bulk composition, but also cracks more readily, especially at lower temperatures.
Since you can't change the microstructure at this point, you may want to induce surface residual compressive streese by shot blasting ( not steel shot).