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I took off the heat shield from the exhaust manifold on my 2003 Nissan Primera while looking for an exhaust leak, and I noticed this yellowish poweder / dust on it:

enter image description here

Any ideas what it is?

EDIT Sept. 19th 2017

So I wiped a section of the yellow powder with my finger and this was the result:

enter image description here

There seems to be another layer of something under the powder which got wiped away at the same time, and the yellow powder itself seems grainy.

I also verified ( as well as I could ) that there is only one exhaust leak which is from the base of the O2 sensor. I stuck a funnel in a length of hose and had my wife block the exhaust with a rag and this was the only place I heard anything, and the change in sound was quite pronounced when she blocked the exhaust.

Robert S. Barnes
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4 Answers4

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IMHO that powder is either:

  • Dust, that got "burned" by the hot exhaust manifold and transformed into some yellow-looking substance.
  • Sulfur, as the color has a striking resemblance to sulfur. The presence of sulfur could indicate a leak and a faulty emission treatment system. There are test strips available to test for sulfur. Should you decide to try a test please get informed about the correct procedure and the risks of this test (lead acetate and Hydrogen sulfide are poisonous)
Martin
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If your engine was running / exhaust manifold hot; the yellow is then zinc oxide. It would result from a galvanized steel manifold which has naturally oxidized. At room temperature the ZnO is white , I forget the temperature where it turns yellow.

blacksmith37
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1

The answer by @Robert is correct.

This yellow powder is Hexavalent Chromium CR(VI), the most toxic form of chromium. It is found on exhaust manifolds as a result of heating stainless steel to 600°C. Do not get it on your skin or inhale it.

Here are additional references:

Regarding the other answers here:

  • Zinc Oxide becomes yellow only when heated. It returns to powder white when cool.
  • Any dust would be burned off on the exhaust
  • Sulfur is also yellow, but would have burned off the exhaust as well

This is not dust, ZnO or S. The yellow powder is Cr-6.

Decontamination

According to a Technical Information Bulletin from Caterpillar and a 2004 Article:

In the event that hexavalent chromium is discovered, Caterpillar recommends following all local guidelines and wearing the correct PPE during the decontamination and removal process. There are multiple methods of cleaning material that can be considered. Once such method is to utilize a solution consisting of 10% citric acid, 10% ascorbic acid and 80% distilled water to convert the hexavalent chromium powder to a trivalent chromium state. Special care should be taken to prevent agitating the powder and creating airborne dust.1

(Ascorbic acid is vitamin C.)

The results indicate that vitamin C could be used in effective remediation of Cr(VI)-contaminated soils and groundwater in a wide range of pH, with or without sunlight.2

Jon Jaroker
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Be wary, this could be hexavalent chromium, which is extremely toxic and a new hazard that has been recently identified with stainless steel and high temperatures, particularly in the presence of calcium-containing insulation.

https://www.eneria.fr/en/hexavalent-chromium-or-chromium-6-cr-6-on-engines/

Robert
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