I haven't done much on the CAD plating chart yet but did talk to the plating guy with respect to black oxide / phosphate coating. Below is from their side:
Black Oxide
Black Oxide Coating process was originally developed during the early 1900’s. The modern bath become commercially prevalent during the later 1930’s and has remained so through today. With little advancement to the actual chemical process, BOI is commitment to continuous improvement in techniques, education, and latest technology has advanced BOI to the top of the Chemical Conversion Coating profession.
Black Oxide process applies to ferrous, non-ferrous, brass and copper alloy. The Black Oxide is a true conversion coating converting iron to the naturally occurring black iron oxide compound called magnetite. The magnetite is produced through chemical reaction as the alloy is immersed into the hot blackening solution. Dwell times depend on such parameters as base alloy, part density, surface hardness, and nature of the heat treated surface. The Black Oxide conversion will not reduce hardness or affect the dimensional characteristics of the finished part. Producing no appreciable dimensional change, it has been documented to be measured at 5 millionths of an inch. The coating becomes an integral part of the base metal surface and does not chip, peel, crack, or rub off. When aesthetics are specified, note that the gloss level or shade of black cannot be controlled. The finish is dependent on the condition or finish of the base material. The desired gloss level may be achieved prior to the Black Oxide process through mechanical means. Machining, polishing, line-graining, abrasive blast, polishing and buffing, all play an integral part in achieving a desired shade or gloss level of black color. Black Oxide is considered as a non-sacrificial coating, meaning it offers a limited amount of corrosion protection and generally is specified when dimensional build-up of a more corrosion resistant finish cannot be tolerated.
BOI performs Black Oxide processes certifying to various aerospace, military, and prime customer specifications:
MIL-DTL-13924 Class 1, 2, 3, & 4 (steel & stainless steel)
AMS-2485 (steel)
MIL-F-495 (brass & copper)
Phosphate Coatings
Phosphate Coatings became popular during the First & Second World War as it was discovered that treating metal surfaces to modify their properties for various purposes including corrosion protection, improving paint adhesion, and improved lubricity. A war-time shortage of tin resulted in phosphate as an option to replace tin plated steel. Phosphate coatings in the original rust-proofing application probably reached an all-time record high during the war particularly in the UK and USA with vast quantities of armament parts owning their durability in service to this treatment. Popularity carried over in post war years as newer applications of phosphate coatings appeared. With few fundamental advancement in the phosphate process since this time, steady advance in techniques have accelerated popularity among metal manufacturers.
Both Zinc & Manganese Phosphate coating are typically a stand alone coating or used as a base for paint or other top coat finishes. Phosphate offers a greatly enhanced bonding on ferrous alloy in addition to being an underlying base coat for added corrosion protection. As a stand alone coating, phosphate provides excellent base for holding rust preventatives, lubricants, etc. A friable crystalline structure is formed which is an excellent anti-galling selection for break in application for shafts, gears, and various moving assemblies.
BOI performs both immersion zinc & manganese phosphate coating processes certifying to various aerospace & military specifications:
MIL-DTL-16232 Type M & Type Z, Class 1, 2, & 3
AMS 2480 (Zinc Phosphate) AMS 2481 (Manganese Phosphate)
I am told and understand that black oxide is used if no dimensional change can be tolerated. ( no build up)
One step up with respect to corrosion protection is phosphate coating. A Zink Phosphate is recommended. If max corrosion protection is desired, a supplemental oil treatment can be added.
He recommended:
MIL-DTL-16232, Type Z, CLASS1 - OIL FINISH
OR
MIL-DTL-16232, Type Z, CLASS3 (no supplemental finish)
Dirk