Remove_Repair_Prevent_Rust

 

 
 

Discussions about various metal stripping methods , including acid dipping, media blasting and other rust removal and rust repair processes.

Chemical stripping via submersion -

Acid is the most common metal stripping method used; usually hydrochloric acid (HCI), also known as muriatic acid. Other methods like phosphoric acid and electrolytic will also be discussed in detail. Now back to acid dipping. I have used this method on a ’65 GTO restoration and although it is effective in removing rust from hidden and hard to reach places, it is extremely difficult to completely remove the acid residue from those same hard to reach areas, especially the seams and lap joints in the sheet metal. Although the acid dipping facility is supposed to pressure wash and “bake out” any residue from the seems, there’s a good chance they won’t get it all and the acid will ultimately reactivate. All it takes is a tiny bit of moisture to get it started again. The problem is you won’t see this, because it'll be in a lap joint that is seam-sealed and painted over. This will provide a nice cocoon for the rust process to lay dormant until it makes a hideous reappearance sometime down the road. A tiny crack in the paint and/or a poorly sealed seem can provide a path for moisture to get in and start the process.

 GTO at the chemical stripping facility
Looks like the local wrecking yard tossing another junker on the pile. Unfortunately, it's Nor-Cal metal stripping unloading my GTO body from the trailer as I looked on in horror. The forklift blades bulged the roof, which had to be heated and shrunk.

 

 GTO body after acid dipping 

 

GTO body   after being stripped. The two step process consisted of a hot bath in muriatic acid, followed by a high pressure rinse.

Note the bare metal has started to flash-rust from the moisture in the air.

 

Another negative aspect of this method is Hydrogen Embrittlement.

Hydrogen Embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and crack following exposure to hydrogen. Steels with Rockwell hardness above C30 are most vulnerable. Therefore, certain parts of your car, such as hood or trunk springs, etc., should be removed before exposure to acid. 

The process can be reversed, however, if the metal has not yet started to crack. Susceptible alloys, after chemical or electrochemical treatments where hydrogen is produced, are often subjected to heat treatment in order to remove absorbed hydrogen. There is a 4-hour time limit for baking out entrapped hydrogen after acid treating the parts. This is the time between the end of acid exposure and the beginning of the heating cycle. Hydrogen Embrittlement relief chart

 

To summarize: Acid dipping your car and then rinsing it off and baking out the residue is not recommended, as it is very difficult to get all the acid out of the seams.

The only way to safely submerse an auto body in acid is to submerse it again in a neutralizing bath. This way all the acid is removed. There are a few facilities that offer this process for auto bodies. These facilities usually offer electro-coating or e-coating, where after the body has been neutralized, it moves on to several other prep stages before finally being e-coated in primer.


-The following is from the Muscle Car Restorations inc. website at www.musclecarrestorations.com/ecoat.html

"Once the vehicle has been chemically stripped and the majority of the metal repair and replacement has been completed, the vehicle is ready for E-Coating. The coating process consists of a series of 13 operations that provide the metal with a surface that is ready for finishing coats of paint. The first eight steps of the process prepare the metal by submerging the body of the vehicle in a series of tanks that wash and then coat it with a rust inhibiting layer. The next step is to dip the vehicle into the electrostatically charged E-Coat tank, which covers every square inch of the interior and exterior of the body and electrically bonds the primer to the surface of the metal. After the body is passed through three more conditioning and washing tanks, it is cured in the baking oven at 375° for 40 minutes. The vehicle is now ready to begin the final body work touches and be prepped for paint."

 

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