I just cleaned much of my engine compartment with Abro Heavy Duty Degreaser, and I noticed that on certain metal parts like the valve cover, thermostat housing and transmission housing a white powder has formed:

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I left the degreaser on for maybe 30 seconds to a minute, and in some cases agitated it with a brush, then pretty thouroghly rinsed it with a water hose.

This powder won't blow off, but it rubs off pretty easily with my finger as can be seen in the picture. This did not occur with other metal parts such as the engine block, and various metal pipes and connectors, nor did it happen on hoses or painted parts. I think the parts it did happen on may all be made of aluminum.

  1. Why did this happen?
  2. How can I easily clean this?
  3. How can I prevent this from happening in the future when I clean?
  • You may be able to polish and clear coat like the do on wheels. Though I have no idea how well it stands up to engine temperatures. Or use a high temp paint.
    – Ben
    Commented May 8, 2016 at 10:55
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    The cleaner oxidized or etched the aluminium.
    – Moab
    Commented May 9, 2016 at 0:16

2 Answers 2


Believe it or not, that is corrosion. You are correct in your assumption that the Valve Cover is certainly cast aluminum. Aluminum can be damaged by many de-greasers that are too aggressive. This happens due to a chemical reaction by the cleaner with aluminum parts. Steel or painted parts will not have this oxidation occur. Usually you can avoid this by thoroughly rising or washing the parts provided that the cleaner is not so aggressive that it is etching the parts.

Most notably, oven cleaners/degreasers contain Sodium Hydroxide which actively eats aluminum surfaces unless deactivated relatively quickly by a thorough washing or rinsing.

Failure to do so results in this oxidation occurring once it dries.

The best way to clean this is with a light dishwashing detergent and a stiff nylon bristle brush. More stubborn areas may need a more aggressive cleaner, however since you mentioned it comes off with a touch, it's likely easily removed with just some mild agitation and elbow grease.

Preventing it from happening going forward, avoid using harsh chemicals on your Aluminum. Find Aluminum safe degreasers, or just thoroughly rinse them before they dry out. Never bake out degreaser on aluminum parts. Many degreasers instruct you to work on a warm motor, this is fine for steel parts, but with Aluminum, you should only clean it when it's cool and make sure to rinse it thoroughly.

  • What I don't get is that I rinsed this pretty thoroughly with the water hose maybe 60 seconds after application. Commented May 8, 2016 at 12:00
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    Sometimes the degreaser can remain in the pores of the metal. This is where a secondary step with a follow up with a nylon brush and some dishwashing detergent comes in handy to help clean up and breakdown some of the cleaner. Commented May 8, 2016 at 12:45
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    This is a great answer. If I could add one thing, I'd suggest to powdercoat the exposed aluminum parts to keep them from doing this again. Cleaning becomes much easier and corrosion is non-existent. Powdercoating is very durable. If you don't want it colored, there are always clear coatings. Commented May 8, 2016 at 13:33
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    Excellent follow up Paulster, I agree wholeheartedly. Painted/Powdercoated surfaces are much easier to clean. Powdercoating is the best as it holds up to engine heat best and is also extremely resistant to cracking and damage. Painting while not as durable is also another alternative. High heat paints applied correctly can also serve you well if prepped adequately. Rattle can paint doesn't hold up as well as the kind applied with a spray gun, but if cared for properly- can provide a decent quality surface, shedding grime and making it easier to clean. Commented May 8, 2016 at 15:38
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    I believe aluminum oxides are very hard & very abrasive, so you definitely don't want to accidentally brush this stuff down into places such as the oil cap, spark plug holes, etc. If it gets in your oil it could kill your bearings. See machinerylubrication.com/Read/1777/aluminum-oxide-oil Commented Oct 9, 2016 at 7:07

Just had this happen to me. Degreaser still messes up powercoating. Just don't use degreaser in my opinion.

  • That's a strange statement ... I've had plenty of powdercoating done to my engine parts in the past. Degreaser, carb cleaner, brake cleaner, nor solvent has had any effect on any of my parts. Powdercoating is some really tough stuff and will resist everything except extreme heat and abrasion. I'm sure it won't stand up to sulfuric acid, either, but who knows. Commented Oct 9, 2016 at 12:58

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