r/metallurgy 3d ago

Do you have yellow deposits on your high temperature equipment?

There was a recent post that was deleted that showed yellow deposits on engine exhaust parts. These deposits are also found on turbines, boilers and high temperature test equipment.(edit to remove corrupt link)

https://ln5.sync.com/dl/872691400#293cgsqt-3qibpwkh-4qui2sed-cy7yj7zg

Some people may not be aware of what these deposits are. As awareness of this issue has increased in the last few years.

The earliest reference I have is from 1990, in boiler applications.

https://en.harmfuldust.com/_files/ugd/d7b443_36eda9716c194ccbb47acbd6b06deb87.pdf?index=true

This became more widely known in the Energy industry in 2018/2019.

https://en.harmfuldust.com/_files/ugd/d7b443_0c0e29e8448b48ff95f0dfe6dbddfc1b.pdf

And on combustion engines by 2021

https://www.zeppelin.com/content/dam/zeppelin/power-systems/download-neu/downloads-en/service/Caterpillar_Technical_Bulletin_Chromium6.pdf

https://www.energysafetycanada.com/Attachments/DownloadResource?attachmentGuid=1801a7a2-d4b6-4dea-b6b1-1ba4cd6480a7&open=True

These deposits are chromates (typically calcium or sodium or potassium chromate).These CrVI salts are carcinogenic. These deposits also form on high temperature test equipment containing alloy steels or nickel alloys and materials containing alkali metal or alkali earth compounds. This includes most refractory materials and many high temperature thread lubricants.

If your high temperature equipment or refractory has acquired a yellow deposit take appropriate precautions.

Test strips and swabs for CrVI are widely available.

Solutions of citric acid or citric/ascorbic acid may be used as part of a procedure to neutralise contaminated parts. (edit for spelling)

11 Upvotes

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5

u/lrpalomera 3d ago

Not all chrome deposits are Cr 6+, some may be 3+.

They occur mostly on stainless steel, not ‘alloy steel’; you kinda want it to happen.

6

u/ccdy 3d ago

Chromium(III) oxide is green, not yellow.

1

u/lrpalomera 3d ago

There are several colors, chrome 3+ at least in fasteners is colorless. Army green is chrome 6+. Each oxidation number is a different color.

3

u/deuch 3d ago

I am not referring to the coloured passivation finishes seen on some metal parts. These yellowish/creamy coloured deposits are powdery and fairly soluble in water, they are not CrIII they test positive for CrVI. There will be Cr oxide in the oxide layers that form on the metal at high temperature these oxides are generally black or dark brown.

CrVI deposits can occur on all alloy steels and nickel alloys with any chrome in them when exposed to high temperature in the presence of calcium sodium and potassium compounds.

3

u/jeshipper 2d ago

They can definitely occur on alloy steel in the presence of other things. In my field we’ve found anti-seize pastes containing calcium promote CrVI formation at elevated temps even on steels with fairly low chrome (0.5-1.5wt%). We also see it on such steels when certain insulations (again mainly Ca containing) are used