Oil and Phosphate Conversion Coating

Black Phosphate Etching and Passivation of Iron or Steel Parts or "Oil and Phosphate"

Introduction

Many bolts on the 1962 Thunderbird are incorrectly painted during restoration. When these bolts came from the factory, they were "dull S2" or "oil and phosphate" coated. The correct term is "phosphate conversion coating" and it can easily be replicated at home using common materials available at a home improvement store.

The Eastwood Company sells a product called a Metal Blackening Kit for $40 that includes a pint of solution that will blacken ferrous auto parts with a dull black finish that can be oiled or clear coated to appear like the original OEM parts coating. The refill bottle of their blackening solution is $17.99 plus shipping for a pint that will make two quarts or $36/gallon. A short Google search led me to an article in Wikipedia entitled Phosphate Conversion Coating. When phosphoric acid, or H₃PO₄, reacts with iron, the phosphate group precipitates onto the surface of the iron forming Fe₃(PO4)₂, or iron phosphate, and releases hydrogen gas. The iron phosphate is black in color but being only several microns thick, offers some corrosion resistance but is commonly oiled to provide additional corrosion resistance. Since the oil will eventually dry out, it can instead be clear coated with a flat or satin clear coat for longer protection. Phosphoric acid is sold as a consumer product to perform phosphatizing of rusted metal in preparation for painting. It is also the active ingredient in Naval Jelly.

The chemicals to perform this process are readily available at Home Depot. Both items are in the paint department, Klean-Strip Phosphoric Prep and Etch and a quart or gallon of denatured alcohol (note: I could not find the Phosphoric Prep and Etch at Lowe's). I had originally used acetone; however, it is rather noxious and evaporates too quickly from the rinse cup. Remember the chemistry rule of thumb: like dissolves like. Water is a polar molecule, meaning is has a slight positive and negative charge and forms weak hydrogen bonds with other polar molecules (see the diagram below). Denatured alcohol is a mixture of ethanol and methanol. Alcohols are also polar molecules. Thus, alcohols are completely miscible with water, meaning they are 100% soluble. This makes denatured alcohol an excellent drying agent. It dries a bit slower than acetone, but works just as well. In contrast, if you were cleaning oily parts, you might choose a non-polar solvent such as paint thinner or xylene.

Materials

Klean-Strip Phosphoric Prep and Etch ($15.78/gallon at Home Depot).
Klean-Strip Denatured Alcohol ($7.28/quart or $15.98/gallon at Home Depot).
Testors Dull Coat #1260 ($4.79 at Hobby Lobby) or equivalent satin lacquer spray coat.
Pyrex measuring cups for etching and alcohol rinsing.
A bucket for water rinsing.
A warming plate to speed the etch process (coffee warmer, thrift store $5 or similar hot plate).
Nitrile gloves or equivalent. I use nitrile gloves from Harbor Freight, $6.99/box.
A pair of goggles while handling acids. You never know when you might have an accident that could splash acid into your eyes.
Oil of some kind for true “oil and phosphate”.

Your initial investment might be about $30 to $50, about the same as a commercial product, however, you will have enough materials to etch every bolt on your Thunderbird that requires it, and it will be true oil and phosphate. It is also quite fun and satisfying to see the results.

Procedure

Let's follow the process with several bolts, removed from the shock tower on the front fender. I have sandblasted these with some rather coarse sand. I had to use a dental pick to clean the inner edge of the recesses in the top of the bolt. Notice the neat NL logo. This was obscured with corrosion until I sandblasted the bolt. On the right is the phosphoric etchant. This most likely differs from the Eastwood kit because it has surfactants in it, thus the bubbles. This is actually a benefit, since it will clean any remaining contaminants that may be on the part. I handle everything with nitrile gloves from Harbor Freight.

Add enough etchant to your Pyrex measuring cup to cover the items to etch. The directions on the bottle for phosphatizing say to dilute 1 part acid to 3 parts water. I used the etchant full strength for my first run. I discovered there was no need to dilute it. Place the bolt in the etchant with an old pair of pliers as the chrome will get etch off of them. The bolt will immediately begin to bubble, releasing a small amount of hydrogen gas.

My garage was 58 degrees this morning and I thought the reaction was going a little slow as I wasn't seeing the results I wanted until about 20 minutes. That is where the coffee warmer came in, to speed up the reaction (Le Chatelier's principal*). I put my Pyrex on it and let it warm. After the solution was warmed (It equilibrated at a boo ut 160 degrees F as measured by a candy thermometer), I was able to get my etch time down to about 5 minutes. You can see how much more vigorous the reaction is now, below left. On the right the bolt is ready to go into the water rinse. Rinse it well in extremely well in water. Also, do not use a good pair of pliers for this technique. The chrome will get etched off of rather quickly.

Following the water rinse, it went straight into the alcohol rinse. A quick blast of air dries it immediately. You can see the heavy iron phosphate crystals on the surface.

In the picture on the left, the bolt on the left is as it came out of the alcohol rinse, the bolt on right has been wiped with a towel to remove the excess phosphate crystals. It is now ready for the Dull Coat lacquer or oiling. If you oil them, the excess phosphate will come off as you wipe them. If you lacquer them, you may want to experiment with wiping the excess material off or not. The bolt on right has just been sprayed with the Dull Coat lacquer.

The bolts in the left picture below show a freshly oiled bolt on the left, and one that has been allowed to soak over night. The third bolt is a black phosphate commercially available bolt without the correct logo.. The middle picture shows the bolt in the shock tower. You can see that it still is retaining a dull steel color compared to the black paint. At the right, the bolt is installed on the shock tower in the engine compartment. The bolts did not suffer any finish damage from the socket.

*Le Chatelier's Principal: If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.