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Author Topic: Phosphate & Oil  (Read 1186 times)
Griff in Fairbanks
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« on: March 15, 2012, 04:37:05 PM »

I've been studying Phosphate & Oil coatings, sometimes referred to as P&O.  It's a corrosion resistant conversion coating put on new and remanufactured parts, such as brake calipers, bolts, etc.

To organize my research and keep track of what I'm doing and how it's going, I've started this thread ...

Conversion coating is a process of changing (converting) the surface of an item through chemical processes to alter its properties.  The conversion coating is chemically bonded to the item, as opposed to film coatings such as paint.  Phosphate coating is a thin (several microns thick) conversion coating applied to iron and steel to reduce corrosion and improve wear resistance.  For us, there are three important types of phosphate coating:  zinc phosphate, iron phosphate, and manganese phosphate.

Zinc phosphate is a light grey corrosion resistant coating that is typically used on items that will remain bare, without any subsequent coating.  It is similar to galvanizing but is thinner and therefore doesn't change the item's physical dimension as much.

Iron phosphate is something I've already been working with, for several years, using Krud Kutter's Must For Rust and PPG's DX579 and DX520.  The process of etching metal surfaces in preparation for painting usually creates an iron phosphate coating that helps the paint film adhere to the item.

Manganese phosphate is a tougher, more durable conversion coating that is used when wear resistance is an important feature of the desired corrosion resistance.  It has a larger grain structure than either zinc or iron phosphate coatings, resulting in a coarser appearance.  The coarser grain structure is well suited to holding oil or wax compounds, especially those containing corrosion inhibitors.

Another name for the manganese phosphate conversion process is "Parkerizing," which is used on guns and military hardware.  If I understand it correctly, Parkerizing is slightly different than what I'm trying for.
« Last Edit: March 15, 2012, 04:39:10 PM by Griff in Fairbanks » Logged

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Griff in Fairbanks
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« Reply #1 on: March 29, 2012, 09:33:09 PM »

The formula:

    3 quarts concentrated phosphoric acid

    9 quarts distilled water

    1/2 cup manganese dioxide

    2 steel wool pads, cleaned and degreased

I used Klean-Strip Phosphoric Prep & Etch.  According to the MSDS, it's 40% phosphoric acid.  Jasco is another brand, although the MSDS says it's made by the same company.  Other sources, which probably list their product as a metal etch or cleaner, will work just as well but I'd read the MSDS to find out the the percentage of acid and to make sure it doesn't include other stuff, such as surfactants or detergents.

Some of the articles I read described using the black stuff inside dry cell batteries (i.e., C- or D-cell batteries) but I prefer a purer compound, without the possibility of other substances.  Chemical supply houses tend to provide forms that are purer than necessary and are usually more (most) expensive.  The best source I found is pottery suppliers because it's also used for coloring pottery glazes.  (I bought two pounds from a local pottery maker.)

Be aware that buying large quantities of manganese dioxide may draw the attention of authorities because it is also used as an oxidizer in fireworks and explosives.  Don't let that hold you back -- the amount you'll need is relatively small unless you're doing P&O on a commercial level.  (Just be aware and prepared to answer questions that may arise.)

The steel wool is available in the paint department of lumber yards, etc.  Finer (00 or 000 grade) is better because you want it to dissolve in the mixture.  (As I understand it, the acid uses the iron in the steel wool instead of stripping what it needs off the part you're treating.)  Do NOT use kitchen pads, such as Brillo, because you'll spend forever cleaning all the soap out of it before you can use it.  We cleaned the pads we used with dish soap and hot water, followed be a good hot rinse, and finished the cleaning with acetone to remove any remaining grease.  (The green cans of brake cleaner are mostly acetone.)
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