Name that corrosion 2

[clay87]

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.

 

[metengr]

Looks to me like biofoul with MIC.

 

[EdStainless]

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

 

[TVP]

My initial reaction is the same as metengr - MIC.

 

[clay87]

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.

 

[metengr]

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

 

[stevenal]

http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm

Since graphite is the most noble in the series, you might want to rethink its use.

 

[stanweld]

My guess is MIC or selective copper plating.

 

[EdStainless]

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

 

[clay87]

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...

 

[Guest102023]

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.

 

[1gibson]

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.

 

[emxhps06]

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

 

[clay87]

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.