MIL-P-16232

Type Z: Zinc phosphate coating

Class 1: Supplementary preservative treatment or coating as specified

Class 2: Supplementary treatment with preservative conforming to MIL-C-

16173 Grade 3 or MIL-L-3150 as alternative for very small parts

Class 3: No supplementary treatment 

Zinc Phosphate coatings are extensively specified for fasteners and other machined parts. The phosphate base provides an excellent substrate for painting and for retention of oils, waxes or other organic lubricating materials. Most zinc phosphated fasteners are additionally oiled to enhance corrosion resistance and to help control torque-tension relationships. Dry zinc phosphate is often used as a base for further processing such as priming and painting.

MIL-DTL-16232

Type M: Manganese phosphate coating
Class 1: Supplementary preservative treatment or coating, as specified
Class 2:Supplementary treatment with lubricating oil conforming to MIL-L-3150
Class 3: No supplementary treatment

Also referred to as Parkerizing, phosphate coatings form an adherent layer of zinc/manganese crystals on metal surfaces through an electrochemical reaction with the metal. It is widely used as a coating for the following reasons:

Prolong the life of organic coating

Provide good paint bonding

Improve corrosion resistance by providing a good base for absorbing and retaining rust preventative materials

Provide an excellent base for holding lubricants and drawing compounds