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Name that corrosion
2

Name that corrosion

Name that corrosion

(OP)
Application: Raw Water Pump Shaft
Material: A276 XM19 (Nitronic 50)
Fluid: Pond water. Slightly brackish (170 ppm Cl- or 1300 um/cm2).
We do inject biocide (i think NaOCl).

I can't recall ever seeing this corrosion in this application with the same material.  We may have a material issue?

Thanks in advance.  

RE: Name that corrosion

Looks to me like biofoul with MIC.

RE: Name that corrosion

Operated continuous or intermittent?
What was used as an assembly aid (oil or grease)?
What alloy was the mating material?

Have you checked chemistry?  Just a Mo spot test would work for me.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Name that corrosion

My initial reaction is the same as metengr - MIC.

RE: Name that corrosion

(OP)
I thought I answered the questions already.  Technical error.

1.  Operated about 3/4 of the time.  May sit idle for 1 week at a time.

2. Assembly aid, if any, would be graphite.

3.  Mating materials: Impeller is NiAlBr (slide 2 top photos).  Shaft sleeves are XM19 (slide 2 bottom left).  Some corrosion where there is no mating matl (slide 2 bottom right)

Note that the upper shaft (can be partially seen on slide 2 top right photo) is the same material spec and does not show this corrosion.   

RE: Name that corrosion

I still stand by my guess. It is only a guess. LOL

RE: Name that corrosion

My guess is MIC or selective copper plating.

RE: Name that corrosion

Stan, you beat me to the Cu comment.
I sort of doubt MIC.  Pictures of the surface fouling could change my mind though.

Either Cu and/ or graphite and a galvanic mechanism sounds more realistic.

Differences in surface condition between the two shafts can go a long way as the source of different corrosion results.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: Name that corrosion

(OP)
I dont know anything about this selective plating...is this similar to selective leaching such as de-alloy/de-aluminification.  What the heck causes this?

I wouldn't expect that a tiny amount of graphite would lead to galvanic problems on such large/heavy parts, but I guess anything is possible...

RE: Name that corrosion

Scrape off some of the deposit and order XRD (compounds) and EDS (elements) testing.  The green suggests copper hydroxide, but it might also be a nickel salt.  Living microbiological material is now impossible, but XRD can give indirect evidence.  Metallographic sectioning of the affected area, if it reveals spheroidal pits, is a strong indicator.  With stainless steels pits can be almost entirely submerged.   

RE: Name that corrosion

Shaft appears to be burnished in affected areas with emery paper, perhaps it was contaminated. Inclusions of other metal from that process may have given the corrosion a foothold.

Burnishing is a common process for pump components, there may be a 0.001-0.002" clearance between shaft and impeller by design, for larger components this makes assembly difficult. Flapper wheel inside the impeller bore, burnish the shaft with a strip of emery cloth. Dust from either or both shaft/impeller could have found it's way into the final assembly, trapped between mating components.

RE: Name that corrosion

looks like galvanic corrosion or some form of preferential corrosion to me.  

RE: Name that corrosion

(OP)
Gibson,
The shaft/impeller may have seen emory cloth for cleaning/smoothing (not necessarily for 'burnishing' but for cleaning/smoothing) so this may be a possibility.  

I'll try to get a sample of the stuff and update this thread.  Thanks.

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