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Ref: DOD-P-16232 (MIL-DTL-16232 now) TT-C-490, AMS 2480, AMS 2481

Why do we use it.
1) Surface preconditioning before prime, paint - enhances paint adhesion, and corrosion resistance of complete system

2) Surface prep for bonding

3) Surface prep before metal forming operations and/or break in surface for new parts, via adherent base for holding lubricants.

4) Improves corrosion resistance mainly by establishing a proper substrate for prime -paint or oil-wax application.

Where is it used:

1) For applications noted above.

2) Military ordnance, automotive surface protection, aerospace prep for subsequent prime-paint-oil.

3) Should not be expected to provide good stand alone corrosion protection by itself without subsequent prime-paint or oil, wax preservation treatment. Bare phosphate coatings only required to pass a 1.5-2.0 hr salt spray test and then this is a challenge sometimes.

When did we start using it?

1) Medieval armorers actually used mild phosphate minerals in water base to improve appearances - beginning of man's distaste for rust?

2) World War I ordnance marks first big use for phosphatizing of steels.

3) DOD-P- 16232 first released on May 2,1951 (first as Mil-C- 16232 then MIL-P-16232, then DOD-P-16232, now MIL-DTL-16232) TT-C-490 first released on March 30, 1961

The What of the process - Chemical reactions:

1) Phosphate coatings-treatments are reactions of Zinc, Manganese or Iron with phosphoric acid (H3PO4) and a steel substrate. 2) The chemical reactions, in order, are:

a)      3 Zn(H2PO4) = Zn3(PO4)2 + 4H3PO4

b)      Fe + 2H3PO4 = Fe(H2PO4)2 + H2(atomic)

c)      Fe(H2PO4)2 = FeHPO4 + H3PO4

d)      Overall is 3Zn(H2PO4)2 + Fe = Zn3(PO4)2 + FeHPO4 + 3H3PO4 + H2 (gas)

(note: Zinc used as example, manganese phosphate chemical reaction similar also. Iron phosphate reaction with steel can follow a more acidic anhydride dissociation of phosphoric acid e.g. P4O10 + 6H2O = 4H3PO4)

Who should know about the process?

a)       Any corrosion engineer worrying about the effects of mother nature

b)       Any metallurgist worrying about hydrogen embrittlement

Hydrogen is evolved from the process although mildly since its non-electrolytic. Probably of greater concern is hydrogen evolved from initial acid clean or pickle operations ( ref data presented Oct '99) The good news on embrittlement is that very modest amounts of hydrogen are evolved, and the crystallographic structure( HCP either columnar-platelet or mixed) of the Zn-Mn-Fe phosphate formed at the surface lends itself to easy diffusion of hydrogen away from the substrate. Extended time room temperature relief (baking) has been found effective. Elevated temperature exposures accelerate hydrogen diffusion and shorten the bake cycle. High production Zn-Mn-Fe phosphatize lines incorporate forced hot air drying in the range of 300-450 deg. F. (Ref Chrysler and General Motors process specs). MIL-DTL-16232 allows either elevated temperature bake @ 210-225 deg. F or room temperature relief (bake) for 120 hrs. TT-C-490 allows either elevated temperature bake at 210-225 deg. F or room temperature relief (bake) for 240 hrs. The most common baking temperature in the metal finishing industry, is 375 deg, F for varying times at temp.

Prior revisions of DOD-P-16232Rev F, (dated Nov 1978 and prior) allowed 325 deg. F bake cycles for type M phosphate coatings. A subsequent revision (Amendment 1 dated 1999) eliminated this option and now only room temperature and 210-225 deg. F bake cycles are used. Prior work with fasteners and ordnance showed modest reductions in corrosion performance of phosphate treatments baked at temperatures above 300 deg. F although this work was based on phosphatized samples with no supplementary oil or wax preservatives or top coating with primers-paints. Prior U.S. Army work on higher temperature baking resulted in the pronouncement that coating dehydration occurred. Numerous metal finishing experts have asked the question "where is the water in a Zn or Mn phosphate coating?" Very light iron phosphate treatments may be susceptible to dehydration but a more probable effect of higher temperature bake temperatures point to changes in the crystallographic structure of the phosphate crystals with/without a resulting change in the adhesion characteristics affecting subsequent corrosion characteristics.

Recommendation: For consistency, stay with 210-225 deg. F bake temperature

Zn phosphate coating, SEM @ 500X           MN phosphate coating, SEM @ 500X

 

 
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