Parkerizing

 One of the first things I did when I got my lathe was begin sifting though stores of old machinist's tools that I had inherited, through my grandfather, from a great uncle who was a machinist at the US Navy yard in Washington DC during WWII. New torpedo designs and such things. Also some old wrenches, some I wanted to use, and some for antique and nostalgic value. Some were my other grandfather's fathers, well over 100 years old.

 After a quick wire brushing I though them in a pot of warm water/phosphoric acid product called Barnacle Buster.  I had a gallon left over from a PS project, and reasoned it would be good at loosening rust without excessive corrosion. In fact, it works very well. Most of the taps, dies, files, and wrenches looked quite good after 30-40 minutes and a light follow-up brushing. A few I left in longer, since they weren't there yet, and that is when I noticed something odd. They were turning gray.

 A little research revealed that what I was looking at was a phosphate conversion coating, related to the Parkerizing finish we are used to seeing on firearms.  Zinc is added; little doubt a few items had some zinc plating that I had not noticed. Manganese is included in the patented Parkerizing formula, as is treatment at an elevated temperature. 

Obviously enough, heat speeds the process and helps remove any traces of residual oil (hot phosphate is a strong de greaser). Concentration helps. But too much speed and the surfaces become rough (more etching) and the evolution of bubbles can lead to streaking and poor treatment of the bottom (also blinded by bubbles). 

The upper wrenches are 100-125 years old. They work fine.

The lower wrenches are nothing special, but are good grade and in fine shape after rescue from a rusty tool box. The photo does not do them justice.

 

I'm writing this up for Practical Sailor, but the short version is this: 

• The recommended concentration, which is a bout a 4:1 dilution, or about 5-7% phosphoric acid.
• Room temperature to 150F. About 120F is probably the best balance of effectiveness, evenness, and minimal etching.
•  A little zinc seems to help. The easy way is to plop an old anode in the pan for 5-10 minutes before adding the parts. More zinc creates a thicker, blacker coating, but it is more prone to rubbing off and seems to reduce durability, so minimize the zinc. 
• You can reused the dregs for many batches over months. Top up with water. Add a little acid if it seems to slow. There will be some sludge, from grease and from the old rust, but it doesn't seem to hurt anything. It settles out and you can pour off the clean liquid. A stainless pot with a lid from the thrift store is good. A hot plate in the shop is better than using the kitchen stove!
• Lightly oil. Mixing any old oil about one in four with mineral spirits gets the oil into all of the crevices, and when they drip off and dry, leaves just the right film. I dunked then in a shallow pan; I had a lot of taps and dies, plus wrenches with internal parts. For general use, I have a jar with a lid and keep a brush and a drip-off pan next to it.

It really does not add much rust protection, but the coating really helps hold oil and the parts I make (lathe , milling, welding) look better.  As for old tools, with a exception of some rusty feeler gauges that were beyond saving, even the files and taps came out sharp, little different from new (treatment for files and taps was more brief). 

The main advantage of this treatment, other than it is dead easy, is that unlike paint or galvanizing, it does not add thickness. Everything works as it did before. Pretty cool.