pitting in CS storage tank 3

[amyh33]

A recent ultra-sonic inspection determined several clusters of pitting in a carbon steel storage tank. Tank holds approx. 200,000 gallons of liquid waste containing organic compounds in water. pH is 11-12, but composition is likely to contain chlorides(to be verified).

The operating company has recommended the tank can still be used at a lower volume to accommodate reduced wall thickness measured in the pitting areas. Also, they have applied a corrosion rate (mpy)to estimate useable life based on the rate of material removal in the pitted areas.

Am I right to be concerned with the logic of applying a general corrosion rate to a pitting problem? What options are there to either monitor pitting, or repair the areas? Tank is only needed for about 1 more year.

 

[metengr]

You don't mention where the clusters of pits occurred. If the pit clusters occurred on the bottom of the tank, it is most likely under deposit corrosion. If the CS tank is to remain in service for only 1 year, and this is the extent of corrosion damage (see below), I would agree with the operating company.

You can monitor corrosion of the storage tank by performing external ultrasonic thickness testing (UT), and to periodically watch for and locate the leaks. If the leaks are in a specific location along the side of the tank, you could patch them externally. If the leaks are on the bottom, this would require you to empty the tank and make repairs from the inside, if the floor is not accessible from the outside.

The rate of pitting attack should not be used to estimate general corrosion attack because they are completely different corrosion mechanisms.

At this point, I would perform some general UT at various locations and compare current readings with the original wall thickness of the tank. If there are significant differences in wall thickness away from the clusters of pits, you could estimate a general corrosion rate from this data (delta wall thickness/time). If you find that the wall thickness has NOT changed appreciably over time, you have negligible general corrosion attack.

What you need to be concerned about is the change in the size of the clusters of pits OR general corrosion attack that could result in a major leak or structural collapse.

 

[stanweld]

You do not indicate whether the tank is internally coated, as I suspect it is since the pitting is clustered. This would indicate localized failure of the coating and potentially a high rate of wastage. You also don't indicate the depth of pitting relative to the tank wall thickness yet it would appear to be great based on recommendations of the operator to operate at a reduced volume. The corrosion rate may well be high enough to breach the remaining wall thickness prior to retiring after another year and thus warrant repair as discussed by metengr.

 

[amyh33]

Thank you metengr and stanweld for your prompt and thorough responses. In answer to some questions raised, the pitting does coincide with the sludge level near the bottom of the tank. There is general corrosion occurring as well as measured by ultrasonic inspection and comparison with original tank wall thickness. The tank is not coated. We do not have an assessment of the depth of the pitted areas.

My biggest concern is that the pitting will continue and weaken the structure faster than the general corrosion rate is indicating (e.g. - could so many pitted areas form that the tank could collaps at that location) - or would we see break-through and leaking first?

Right now the recommendation is to perform ultrasonic inspection every 6 months to monitor. I am determining if this is reasonable given the general corrosion rates calculated from the last 6 month inspection.

 

[metengr]

amyh33;
Your re-inspection interval seems appropriate. I would expect that you will have several large leaks before any structural collapse would occur - leak before break.

 

[SJones]

Two things:

1 The operating company needs to define its leak policy (or environmental impact and official regulations regarding leaks). If it has a no leak policy, then that will drive how the pits are treated.

2 If trying to justify leaving pits of unknown depth, somebody must get a handle on the specific corrosion mechanism to have the basis for predicting the remnant lifetime.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[amyh33]

Leak policy is that it cannot happen! Tanks do have 100% secondary containment, just in case.

Pit depth was measured and compared with allowable depth per API 653. This showed the remaining tank wall thickness was greater than the minimum requirement per API 653 for "scattered pitting".

Root cause analysis is underway and is pointing to a galvanic cell being created within the tank by a sludge/residue layer (coincident with the pitting areas) and a highly suspect chloride-containing liquid.

Path forward is to reduce the volume in the tank and monitor more frequently with daily exterior visual inspections and monthly ultrasonic inspections. Ultimate life needed of tank is less than 1 year.