Gunplumber's Notes

GUNPLUMBER'S GUIDE TO HOME GUNSMITHING:
PARKERIZING OVERVIEW
(revised 09-26-99)

INTRODUCTION

The gun finishing process commonly known as "Parkerizing" can be a fun and exciting process for both the home hobby gunsmith and the retail gun shop. Gunplumber's Guide will give you a better understanding of the process, and help you to decide if it is a venture you wish to undertake yourself. Arizona Response Systems offers high quality "Parkerizing" for firearms. Check the website for options and prices. You may have noticed my use of quotations around "Parkerizing." Parkerizing is a trade name owned by the Parker family, which developed the process in the late 1800s. It is currently owned by Parker-AmChem Company, but has become so common in today's language (like Xerox or Teflon or Frizbee) that it is frequently used erroneously as a generic term. The process covers a wide variety of phosphate deposition coatings. The most common for the military firearms industry are zinc and manganese phosphate, with iron phosphate coming in a distant third. The phosphate deposition process leaves a non-reflective coating on ferrous metal. This coating is an adequate protection against corrosion and abrasion in its own right, but additionally acts as a sponge to absorb oil for greater corrosion resistance and is an excellent primer coat for paint. The process can be adjusted to result in a variety of grain structures, from coarse to superfine, and in a variety of coating thickness usually measured in weight/surface area. Most commonly, it is an immersion process, but it can also be applied in a spray. Phosphates have excellent anti-galling properties as well. You cannot phosphate stainless steel, aluminum, brass, or copper. Best results are on carbon steel, "cold rolled" steel of the type found on most guns. Steels with high nickel content such as some older Winchester parts, may not take as well. Zinc phosphate is a light to medium gray, depending on hardness of base metal,. Manganese phosphate runs a medium to charcoal gray, as does iron phosphate. The latter two are a heavier grain structure and for that reason, even using a grain refiner pre-treatment, will usually give a coarser finish than zinc Nevertheless, the color of manganese phosphate is most popular with our customers.

REASONS FOR USE

Phosphate coatings are desirable to the home gunsmith because they are relatively easy to do, compared to hot bluing or baked on coatings, and relatively benign. They require lower temperatures (>200 F.) then bluing, do not "spit" like caustic salt bluing, and the acidic sludge is easy to collect for safe and proper disposal. Another benefit of phosphate coating is that the low operating temperature allows parts permanently bonded to plastics to be coated without melting as they might in baked-on coatings and hot bluing. HK cocking handles and retractable stocks, and StG-58 cocking handles are examples. One application of phosphate coatings that has been particularly useful in my business is to test questionable parts. There are some receivers and parts that are advertised as originals but are actually welded together from demilled components. Some weld jobs are beautifully surfaced and nearly impossible to detect visually, but if the item is phosphated, there will usually be a shift in color on the welded spot. Softer metals show up darker than harder metals and it is rare that a welded area will have the same color as the rest of the part. Silver soldered parts and brazing will also show up, where the braze line may have been covered by a different coating.

PROCESS DESCRIPTION

In its most basic form, an item to be phosphated is physically cleaned and degreased, sand blasted to bare metal, cleaned again, rinsed, dipped into a grain refiner (but NOT rinsed) and submerged in a heated tank of phosphoric acid and zinc (or manganese, etc). The acid starts a pickling process on the surface of the metal, releasing gas and making a gas sheath of tiny bubbles around the metal. The dissolved metal come out of solution and adhere to the metal. Because there is some microscopic etching of the metal, the process is sometimes referred to as a "conversion process." The acid pickling the metal does microscopically dissolve the metal and use the released hydrogen to make the phosphate coating integral with the surface of the metal. I call phosphating a "deposition" process, because the coating thickness can be measured as greater than the original part. It does differ slightly from a deposition process such as painting or electroplating where another substance is laid over the surface. Those grains that do not adhere, called "flock" fall to the bottom of the tank and create an unusable "flocculent sludge." The sludge must be periodically filtered out of the tank. When the part stops bubbling (gassing) it is removed from the tank, rinsed, inspected, then immersed in a water-displacing oil and hung to dry or wiped down. If the part is to be painted, it is dried with compressed air and not oiled. The type of oil causes a change in color of the phosphate finish, darkening it (called "curing"). Color changes can also be induced through long-term storage. Cosomline (not a petroleum product, but a lanolin-based organic grease) and other gun-care products, as well as surface oxidation and years of cleaning and rubbing, cause phosphated surfaces to take on a greenish brown tint. This color, which I will call "patina" is one many gunsmiths wish to duplicate in restoring rifles, such as the M1 Garand.

EFFECT ON RIFLING

Because there is etching, however slight, custom, match grade barrels should probably be plugged. Use a rubber stopper in both ends, and if you have trouble keeping it sealed, use a brass threaded rod inside the barrel (steel might mar) and then slide your rubber bung over either end and tighten with a small nut and washer. Chrome-lining does not take phosphate coatings and the crud that appears on the inside of chrome lined barrels can be cleaned out with patches. Standard barrels will get a bit of crud in them, but cleaning and firing a few rounds will restore the inside finish to original.

BASIC EQUIPMENT

Cleaning Solvents
Cleaning Container
Sand Blaster
small parts basket
rinse tank (any material)
pretreatment tank (any material)
pretreatment chemical
processing tank (stainless steel preferred)
phosphate solution
heating unit
thermometer
safety equipment (gloves, apron, goggles, baking soda, etc)
oil tank
water displacing oil
drip area (towels, hangers, etc)

PROCESS DETAILS

Surface preparation is best obtained through degreasing the part in a solvent such as lacquer thinner, acetone, or a heavy scrubbing in hot water with detergent. Stamped parts that have been brazed together will often leach oil from between the joints, so the process may need to be repeated several times. Avoid solvents that leave a petroleum-based film on the parts.

Sandblast the part with an #80 aluminum oxide. You may also bead blast or skip this step altogether, however I have had best results with sandblasted parts.

Clean the parts again by dipping in lacquer thinner. Have parts in baskets or wired. Avoid touching with your fingers as the oil from your skin will mar the finish.

Rinse parts again in hot water (boiling if possible), but only when ready to immediately process, to avoid flash rusting.

Immerse parts in the pre treatment (rinse conditioner - usually a titanium dioxide solution) per manufacturer's instructions - usually a few seconds. Do not rinse - put directly into the phosphate bath.

The phosphate bath is best heated by an industrial immersion heater. This is very expensive. Heating over flame or external electric element can cause hotter spots in the tank, which will cause a hardened buildup of sludge. For low volume, it may not be cost-effective to use an immersion heater. Also, a stainless steel tank will not react to the solution as a cold-rolled steel tank will. However once a layer of phosphate has built up on the surface, it becomes non-reactive.

Bath temperatures are going to depend on the particular brand of solution, coating thickness desired, and immersion time. This is where the technical side of phosphating far exceeds the requirements of the home gunsmith. Typically, the hotter the solution and the longer the immersion time, the heavier the coating. However too high a temperature will increase the amount of phosphate expended that does not adhere to the surface of the metal, but falls away as flock. You must not allow the solution to boil. I have used a variety of brands, and find that 180 F. for 10 minutes is a good starting point. Raise the temperature (max 190 F) or increase the immersion time. It is of no value to keep a part in a tank after the gassing has stopped, as no further reaction will occur. After 15 minutes, you can remove the part even if minor gassing continues, as longer immersion may start to etch the part.

Remove from tank. Rinse in clean, cold water. There may be a thin layer of white flock on the parts, which can be rinsed off or scrubbed off with a soft brush and rinse water. Inspect the part. If there is a minor flaw, the specific area can be cleaned and blasted again, then the whole part returned to the solution. Only the re-prepped portion will undergo a reaction. If a large area is flawed, then it is best to blast the whole part and start again.

If the parts are going to be painted, blow all water off with an air hose. paint as soon as possible because the virgin material is subject to a dusting of rust if not oiled. Sometimes this dusting is caused by impurities in the rinse water oxidizing, however unless you have an inexpensive supply of purified water, it is not significant enough a problem to justify the added cost of super-filtered/distilled water. Immerse the part in water displacing oil. I prefer WD40, although a number of special "curing oils" are available.

Hang part to dry, or dry with lint-free towels. I prefer newspapers for the bulk of the oil, and blue paper shop towels for final wiping.

TIPS

After initial phosphate mixing per manufacturer's instruction, replace water lost through evaporation and steam. Mixing a little extra solution beforehand can help to top off a tank after it has been filtered. If coating thickness seems to decrease over time, add more of the concentrate solution.

Typical phosphate solution concentrate is about $25 per gallon and enough to do 20+ rifles. If an accurate measure of the reactiveness of the solution is desired (total acid to free acid ratio), you will need to buy titrating equipment - see advanced techniques section.

An 18 gauge black iron tank from Brownells lasts me about 6 months of heavy use before the acidic solution eats through the tank. The life of the tank can be greatly increased by storing the phosphate solution in plastic buckets when not being used.

Practice with handguns, magazines, or small parts, on the kitchen stove with a $20 stainless stock pot or porcelain canning pot. It is easier to learn with small containers and you have less up-front investment if you decide against continuing.

A new solution will not produce an appropriately thick coating until conditioned, either by processing non-important parts, or the one-time addition of a little steel wool or iron powder, usually in a low temperature tank for about an hour - see manufacture's suggestions for details.

Five gallon buckets make good rinse tanks. You can fill buckets from your bath tub, which produces adequately hot water if you do not want to heat your initial rinse tank to boiling.

To do a small part, a coffee cup with enough solution to cover the part can be heated in the microwave. I've never had a submerged metal part cause arcing in the microwave oven, but you may wish to heat the solution first, then drop the part in. Keep covered with a paper plate to avoid splattering solution inside oven, if it boils. Clean cup well before using for food.

A Tupperware butter cover works well for StG58 cocking handles (plastic-metal bond)

Plastic planters from your garden shop can be used for rifle rinse tanks or grain-refiner tank.

You can skip the grain refining step, if you don't mind a slightly coarser texture

You can go directly from the blaster to the phosphate tank if you don't mind a coarser texture, increased sludge creation, and slightly erratic coating thickness. I do this when I process my shop tools that are starting to rust or other applications where cosmetics are unimportant.

If using a gas heater, keep the fans away from the flame as it will make it hard to maintain a consistent temperature.

A 40" Brownells tank fits diagonally on many kitchen stoves. Keep stirred as the heating elements will not hat the tank evenly.

If you are setup to do handguns, and need to do a rifle-length part, repeatedly pouring the hot solution over the rifle barrel, held vertically, may work in a pinch. You can also transfer your hot solution to a plastic planter or fiberglass (wallpaper supply store) tank to hold the rifle. In industry, large parts are often coated by a car-wash type spray process.

SAFETY ISSUES

Have excellent ventilation. The steam from the phosphate bath is slightly corrosive and can oxidize tools and even etch contact lenses with continuous exposure.

The phosphate solution is poisonous

The phosphate solution is acidic. If you get it on you, rinse well and neutralize with baking soda. Wear gloves goggles, and an apron. I don't bother with the apron and only use gloves if I have to fish something out of the bottom of a hot tank, but eye protection is a must. Follow the manufacturer's specific precautions. I've fished parts out of a 140 F tank with bare hands to no ill effect, but people have different sensitivities to the acid. I have also spilled a large quantity on my pants and received minor burns - like a sunburn, which no amount of rinsing eased - baking soda cooled it off.

Did I mention ventilation? Los of it - fans and more fans!

CLEANUP

As long as your flocculent sludge is not settled on your parts, it will not affect the process. However it eventually should be filtered out. After cooling, the sludge will settle to the bottom and the clean solution can be siphoned off the top. I siphon through large coffee filters. Keep your wet sludge in an open bucket. The water will evaporate off and minimize the amount of hazardous waste that must be properly disposed.

COLOR CHANGING

Preheat an oven to 400 degrees. dip your gray part in a medium weight oil, like motor oil. dry thoroughly. place part in oven for 20 minutes. Inspect. Increase time if necessary, checking every 5-10 minutes. As the oil burns, it darkens the surface of the metal, aging it to a green-brown tint. Consider the ramification of doing this in your kitchen oven (smell, angry wife, etc.) before trying. Wiping with Cosmoline, grease, or a variety of other petroleum products and dyes can affect the finished color. Experiment. I find Castrol amber wheel bearing grease to give a nice patina.

ADVANCED TECHNIQUES: TITRATING THE BATH (draft)

Two 50ml graduated burettes in appropriate stand. 100 ml conical flask 10 ml pipette .01% Chromic acid 50 ml glass or clear plastic dropper bottle Normal 10 sodium hydroxide reagent grad phenolthalene Bromophenol blue 75% technical grade phosphoric acid sodium bicarbonate or sodium hydroxide )(flake or pearl) Titrating for free acid content: Put 5 drops of phenothalene into 10ml burette of phosphate solution. Add additional drops until solution turns pink and holds color for 15 seconds. Total ml is your free acid number. Titrating for total acid content: Repeat, starting with 5 drops of bromothalene blue. May take 20-30 drops to change color. Total ml is your total acid number. total acid : free acid 4-6:1 5:1 ideal To increase free acid content, add 75% technical grade phosphoric acid, or more of your concentrate. To decrease free acid content, add sodium hydroxide or sodium bicarbonate, or dilute tank with water.

GUNPLUMBER'S GUIDE TO HOME GUNSMITHING: PARKERIZING OVERVIEW (revised 09-26-99)