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Aluminum crankcase darkened after cleaning


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03-19-16, 07:32 PM   #1  
Aluminum crankcase darkened after cleaning

I soaked an aluminum crankcase off a Honda dirt bike in POR 15 cleaner/degreaser for a couple hours washed the case with dish soap and hot water. The case is now dark gray and when I rub the case with a towel black residue rubs off. First, what did I do wrong, second, how can I clean the case so it is a nice bright silver color like it was before I cleaned it?


Last edited by mossman; 03-19-16 at 08:14 PM.
 
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03-19-16, 10:38 PM   #2  
I dunno what the degreaser you used is, but the only way to remove residue or restore the bright finish is to rub it with a polishing compound after removing the residue. You can do it by hand or use various tools...

Who cares...it's a dirt bike right? You seem more concerned with appearance over function sometimes.


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03-20-16, 05:00 AM   #3  
The instructions for POR15 state that it can be used on aluminum only for brief periods. I assume it's an alkali (base) cleaner and it attacked the aluminum.

 
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03-20-16, 06:55 AM   #4  
I'm not overly concerned with the appearance. I'm concerned with the black residue inside the cases and whether or not it needs to be removed prior to putting everything back together. I didn't see the warning about only using it on aluminum briefly. I soaked it for two or three hours. You're saying the only remedy is to use an aluminum polish? How am I to get every nook and cranny inside a crankcase? Seems like some sort or chemical dip would be more appropriate. Maybe diesel or kerosene would clean them up?

 
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03-20-16, 07:31 AM   #5  
Unfortunately there is no quick fix to restore what you have.

Most aluminum objects are either painted to look like aluminum or are polished and have a clear coat finish on them.
You will have taken off whatever finish was applied.
To restore it you need to either manually polish it to the luster you want then clear coat or paint it with an aluminum look paint.

I would suggest you manually polish it to an reasonably even finish and then paint with an aluminum look paint.


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03-20-16, 08:33 AM   #6  
Polish and paint the inside of a crankcase???

Is there any harm in leaving it as-is? It really doesn't look that bad. My concern is having the black residue bleeding off and mixing with the oil inside the case. Should I at least polish the gasket mating surfaces?

 
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03-20-16, 08:47 AM   #7  
I wouldn't worry about the darkening. You might have some minor porosity or pitting depending on the quality of the casting but there is nothing you can do about it now. Removing the oxide/blackening will also be removing metal which you probably don't want to do in some areas inside the engine. I'd just give surfaces where bearings or bushings go a good wipe with a rag to remove any loose material. Anything that happens to come loose when the engine is running will hopefully just get blown out. If you really want to clean the inside of the case I'd blast it with walnut shells.

 
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03-20-16, 08:50 AM   #8  
Okay. I'll probably just leave it as-is. However, since I only cleaned one half of the case the other half doesn't match. I think I'll get everything back together first then clean the other half with the same degreaser so they match. Or is this a bad idea?

Here's a pic. Top half hasn't been cleaned, bottom half has:


 
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03-20-16, 02:01 PM   #9  
Someone with experience in the field told me boiling water and vinegar should work.

 
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03-20-16, 04:10 PM   #10  
Aluminum is not very resistant to acids and bases. It is susceptible to being etched/eaten by both. The previous soak etched the metal. Doing a soak in acid will do similar though on the opposite end of the pH scale. Both will etch (dissolve) the metal. You may end up with a different color using acid but it will not undo what was done by the base. A quick wash in vinegar could help neutralize any remaining base from the previous soak but simply washing the part thoroughly in water will do the same thing with much less risk of further damage.

 
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03-20-16, 06:25 PM   #11  
I'll try soap and water again then. Okay to use a blue scotch brite pad? And I guess I should ask, what is the proper way to clean an aluminum crankcase for future reference? Parts washer solvent followed by soap and water?


Last edited by mossman; 03-20-16 at 07:22 PM.
 
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03-21-16, 05:02 AM   #12  
You can use a Scotch Brite pad but that is basically sanding and are removing metal. The black you get on your hands when using the pad is aluminum that you're polishing away. So, go easy on bearing or other critical surfaces.

The cleaner you picked can be used. I normally use Purple Power which is similar. You just can't let aluminum sit in it. Wear rubber gloves. I usually use a mixture diluted 50/50 with water. The more mild the solution the easier it is on the metal but it also doesn't cut grease as well so it's a bit of a trade off. The cleaner with a brush can get the grime and oil off pretty quickly to minimize etching. Then rinse the parts thoroughly with water. I like to dunk or make sure I keep the entire part wet with the cleaner so it has a uniform appearance.

 
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03-21-16, 07:42 AM   #13  
Thanks for the tips.

Come to think of it, I used about a 1:10 ratio and it still etched it. Lesson learne. That seems to be par for the course for me lately--I research the crap out of something, ask questions on forums, etc. and there is always some minor detail that no one provides, then I screw something up and all of a sudden everyone chimes in with "you shouldn't have done that". I'm not referring to this forum, I'm referring to another motorcycle-specific forum I belong to. I should have asked you guys first, but really didn't think soaking aluminum in a diluted degreaser would have done this.

 
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03-21-16, 03:43 PM   #14  
I don't think you screwed anything up. It's just darkened. I just wouldn't knock myself out trying to get it back to shiny aluminum color especially in precision areas. I'll have to do a test in my shop tomorrow. I'll soak some aluminum in Purple Power, get it to darken then see if an acid bath will easily get it back to silver. I have a jug of Alumiprep (an acid) intended for etching aluminum before painting. If it works it would be a lot easier than a Soctch Brite pad.

 
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03-21-16, 08:21 PM   #15  
That would be great. I'm looking forward to the result. I'm currently discussing the matter with my brother in law to see what he wants to do. He couldn't care less about appearance, but neither of us knows if it is okay to use the case as-is. What if I clean the gasket mating surfaces only and end up with a leak once everything is back together? Or what if etching the aluminum is going to throw something out of tolerance and cause a catastrophic failure. I honestly think it would be fine, but a new case half is only $227 so not too bad for piece of mind. Let me know how your experiment goes.

 
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03-22-16, 04:43 AM   #16  
Is this the stuff you have? Sounds promising. AlumnaPrep - ALMP-33GL - Alumiprep - Chief Aircraft Inc.. Although it sounds kind of nasty. I have a respirator, gloves, goggles, and apron, but I would need to seek guidance on proper use and disposal. Fortunately I have some contacts at work that should know.

 
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03-22-16, 05:46 AM   #17  
You can also look for it a local paint and auto body suppliers. There is Alumiprep 33 which you found and you can also use Metalprep 79. Both are acids that clean and etch the surface of aluminum usually in preparation for anodizing or painting. My local NAPA store carries it and it comes in quart bottles so you won't have to get a full gallon. The trouble with doing this is it really opens the metal up to receive... something. So, if you don't paint or clear coat every finger print and smudge of grease will really show up.

If you want a gold/brownish tone after etching with Alumiprep or Metal Prep you can treat the aluminum with Alodine. Here's a video of a guy going through the process and you can see the color you end up with. It's the only other thing I can think of other than anodizing or painting.

 
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03-22-16, 06:02 AM   #18  
Here is aluminum after 5 minutes soaking in Purple Power (a base) cleaner. At the bottom you can see the shiny untreated aluminum and at the top is the haze caused by the etching. I'm hoping this alloy turns black like yours or I'll have to try a different piece.


 
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03-22-16, 07:11 AM   #19  
I have no trouble working with Alumiprep though I do wear rubber gloves. You obviously need to be careful not to splash it in your eyes but it's not strong enough to smoke or emit vapors that cause irritation. It is no where near as nasty as Muriatic (hydrochloric) acid.

70 minutes in and my first piece of aluminum is still silver though etched more. I'm throwing in other alloys to see if I can get one of them to blacken.

 
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03-22-16, 07:20 PM   #20  
Try using a piece of cast aluminum alloy?

I soaked and scrubbed the case again this evening in vinegar (the strongest I could find) and it lightened up a shade. I think I got most of the black residue off, but it is still a dull dark gray. If I can get some reassurance that the case is fine to use as-is and the darkening is only cosmetic, then I think I may take a chance and button the motor back up and hope for the best.

Current condition:




 
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03-23-16, 04:53 AM   #21  
I didn't have any cast aluminum but I did end up soaking three different alloys. One showed pretty severe pitting after three hours while the other two were in much better shape.

I would stop trying to go for a "color". Anything and everything you do to those castings is removing metal. I'd give the bearing mounting surfaces a light buffing with a Scotch Brite pad then install the bearings to make sure they fit nice and snug. If they fit tight then I'd clean up mating/gasket surfaces and put the thing together. If the bearings are a bit loose you'll have to decide if you want to Loctite them in or trash the case.

 
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03-23-16, 06:04 AM   #22  
I cleaned the gasket surfaces with a blue "no scratch" scotchbrite pad and they are nice and smooth (always were). The more I look at the case the better I feel about using it. All the surfaces are nice and smooth and other than the color it doesn't appear that any damage was done (other than the obvious etching of a minute amount of metal). I think it will be okay. If I do anything else, it will be a soda blast to the external surfaces only so it's a closer match to the other case half. I appreciate all the input!

 
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03-24-16, 09:56 AM   #23  
If I do decide to soda blast, do I need to apply a protective coating to the case afterwards?

 
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03-25-16, 06:01 AM   #24  
You've opened the pores and roughened the surface so every bit of dirt and oil is going to get into the metal and be difficult to clean. Clear coating and even paint are option if you're only talking the lower case where the heat is relatively mild. Another option I would consider is a cold or electroless black oxide. There are several that are electroless (don't need to apply an electric current to apply) and you've already done the case etching so your part way there already.

 
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