316L wharf pipework corrosion

[CSGWB]

Approximately two years ago we installed a diesel refuelling line fabricated from 316L which runs under an existing timber wharf approximately 1meter above the high water line. On a recent inspection, red staining and areas of corrosion were noted along the length of the pipe only on the sections under the wharf. Does anyone have any idea what would cause this?

 

[Metalguy]

Red staining of SS is frequently associated with MIC. Do you have any pitting on the OD?

 

[CSGWB]

Metalguy,

All with have is photos to go by as the site is remote, but there does appear to be a small degree of pitting in places.

CSGWB

 

[Metalguy]

With 304/316 and the L grades, what appears to be tiny OD pitting can turn out to be large subsurface cavities! It's also REAL embarrasing when the NRC resident inspector is the one who discovers your leaking pipe with the above problem in a safety-related piping system-BEFORE you actually fire up a big nuke.

 

[unclesyd]

There are a couple of other things to consider. I've seen freshly treated wood, treated with CCA @ .45 retention or higher, that is still wet start to corrode S/S steel 304 S/S more so than 316 S/S.
Any iron from hangers,fastners, etc. that gets on the pipe would help Metalguy's bugs gain aquatters rights.
As he notes they are insidious as a small pitted area will lead to a condominium.
You didn't say whether the line is 1 meter above salt water or fresh water. I assume saltwater as you used the term wharf. 1 meter is in the splash zone, the actual worst place to have a piece of S/S.
The line needs to be inspected closer to determine exactly what going on.

 

[mcguire]

The top of the pipe may well be the worst because splashes can evaporate and concentrate there, leaving a very concentrated salt solution. There is probably little rain reaching the pipe to cleanse it under the wharf. The top seems like the logical locus of corrosion.

 

[azertyuiop]

I remember a similar situation , were corrosion did start with iron tools falling on the SS pipe during a poorly supervised and uncareful construction phase , leading to numerous carves in the OD pipe SS protection layer by the construction tools , contaminating it with microscopic iron parts. The sea water splashes finished the work , by corroding those iron pits further inward up. The whole shebang had to be broken up , reconstructed with SS support hinges and SS adequate erection tools , and protected from sea water splashes by use of a protection wall made of wood fibreboard panels , marine quality.