What is "parkerized"?

[flabbydan]

Besides color, what is the difference between a parkerized finish and regular bluing?

[Iowegan]

Parkerizing is a coating not much different than paint. It does offer excellent rust protection and is cheap to apply. Blued guns require a lot of prep work (buffing) and are placed in a tank of boiling bluing salts. The salts ionize the surface of the metal and turn it blue (closer to black).  The ionized surface provides some rust protection but is more for cosmetics.  Of the two, bluing is way more expensive and not near as durable to the elements.  Parkerizing was designed for military guns where durability is more important than pretty.

[gunnut69]

parkerizing is an corrosion of the steels surface by a phosphate solution.  The green, gray, or other color is added by staining the layer of corrosion built up on the surface of the steel.  The biggest advantage of parkerizing over bluing is it's faster(less prep work) and the thinker coating allows for more protecting oils or waxes to be retained..  It's disadvantages are it's relative softness when compared with bluing..  There are 3 types of bluing..cold bluing(actualy a form of parkerizing), hot salt bluing, and rust bluing.  Cold bluing usually operates in much the same fashion as parkerizing.  It's used mostly for touching up wear spots but some of the better cold blues are nearly as tough as the best parkerizing.  Hot salt bluing uses metalic salts in a water based solution to create an oxide coat on the steel.  It's really black and a cousin of the black oxide finish found on some drill bits.  Its a bit tougher than parkerizing but not nearly as tough as a good rust blue.  It is fairly quick to apply and once the tanks are set up it's real advantage becomes apparent.  It's quick and cheap to apply.  The steel is brought to the level of polish desired and immersed.  Usual bluing times will be less than 15 minutes.  Rust bluing consists of creating a thin layer of rust (ferris oxide/red rust) on the surface of the steel.  This is then converted to ferric oxide/black rust by heating in boiling water.  Many thin coats are applied to acheive the level of darkness desired.  Depending on the process this can take from several hours to a week.  Time is the bigest cost although the resulting finish is many times more wear resistant than any other finish.  All of these finishes are the result of converting the surface of the steel to another compound which is then either colored or is the color we desire.  Time to apply, durability and cost are some of the considerations. None of these finishes will totally protect the steel they color but they will help retain oils and waxes which will protect the steel.  Of these finishes rust blue is the best protection as it's the hardest and thus will last the longest.  Since it's already rust, the steel beneath is protected.  Parkeizing is the worst protection.  It's composed of relatively soft compounds but is quite thick in depth.  This allows it to retain more of the oils and waxes which can protect the base steel.  Parkerizing material alters the top layer of steel, paint doesn't do that.  The military uses parkerizing on steel because it provides a good cost efficient coating for steel.  Not necessarily the best coating, just the most cost effective.  It's much thinker than hot salt bluing but is also much softer..  both hold the oils and waxes in place to provide corrosion resistance.  Rust bluing itself provides a large measure of rust resistance and is many times harder than eith parkerizing or hot salt bluing.  But rust bluing is quite slow and very expensive..

[John Traveler]

Gunnut69's good basic explanation of the differences between bluing and parkerizing are mostly correct, but with a few small changes.

The various forms of "bluing" are only rust-resistant, and not rust proof at all, as we have all seen active red rust through a beautiful blued surface.

So-called "parkerizing" is the phosphate coating that Gunut69 described and is a modification of the steel surface.  Only ferrous (iron-based) metals can be phosphated.  Parkerizing was the patented trademark of the Parker Metal Finishing Company prior to WWII.  Since then, zinc and manganese phosphate finishes have been developed that are used extensively for military and industrial applications.  They differ from the original iron phospate coatings in that the grain structure is finer and more complex, thus offering greatly improved corrosion resistance and ability to hold sealer top coats.  Manganese phospate in particular is good for salt-spray resistance, and is standard for military weapons exposed to extremes of humidity and temperature changes.  Phosphating is also commonly used as a base  undercoat for finishes that add protection and beauty.  Epoxy and powder coatings over phosphate are used by European military weapons makers such as H&K (GIAT), the former Soviet Union, and many NATO members.  They are less well known in the US military because of the "traditional" phosphating of small arms since after WWI.
 
Other phospating applications include industrial machinery, automotive parts (body panels, engine components), and

[gunnut69]

JohnT is right in that none of these finishes is rust proof.  The areas of a rust blue that have rusted and been converted to ferric oxide are rust proof as they have already rusted..  The problem is when there is not a total coverage which in the real world is nearly always..  I tried to point out that even though none of the finishes are rustproof they are of varying hardness and thus wear differently.  Since what they really provide are a place for the rust preventative oils and waxes to live, a longer wear life is an advantage..  I still believe that a really well applied rust blue of a fine grain character is the closest we can come to a 'rust proof' finish.  Other than some of the plastics and powder paints..