Protective Finishes - Zinc Phosphate Type
|Publication Date:||1 January 2019|
This standard covers the materials and performance requirements for zinc phosphate treatment and posttreatment. This coating is typically applied on engine and transmission components and on applications where minimum corrosion protection is required.
Material Description. This finish is composed of a zinc phosphate conversion layer, providing the substrate material with improved surface adhesion characteristics for subsequent application of lubricant, paint, adhesives, etc. This standard also defines the requirements for various supplemental treatments with zinc phosphate as shown in Table 1. The different post-treatments are identified by different Code suffixes.
Symbols. Not applicable.
Applicability. This coating is suitable for internally threaded parts, parts with internal drive recess, externally threaded and non-threaded ferrous parts where minimum corrosion protection and freedom from hydrogen embrittlement are required.
Use of this coating on threaded surfaces and/or bearing surfaces of joints could affect the torque-tension relationship. It is recommended that a torque-tension study of the fastener joint be performed before releasing this finish on any new application.
Coefficient of friction values in this specification are determined at ambient temperatures. New applications for this finish should be tested to confirm performance under application specific operating conditions.
Finishes with integrated lubricant (Codes G and H) conforming to the requirements of this specification can be used in the application range of -50 °C to +120 °C. For applications outside of this temperature range, consult your local Material/Fastener Engineering group.
Finishes with oil coatings are not recommended for usage against plastic surfaces. It is recommended that pre-applied adhesives, sealers and nylon-type patching elements be applied before supplementary treatments (Codes A, B, G, H, W, and Y).
Code D (no supplemental coating) finish provides minimal corrosion protection. The Tier 1 supplier must take suitable process and logistics considerations to ensure parts do not prematurely corrode. This may include adding a rust preventative coating after phosphating. This process shall be presented during the Advanced Product Quality Planning (APQP) activities.
This coating process generates a small amount of hydrogen in both the acid pickling and the zinc phosphate process. This coating has high permeability, which allows for the effusion of hydrogen after coating. See 3.8 for precautions against hydrogen embrittlement.
This coating process exposes parts to acids and may generate surface pits on metal surfaces. Generally, the higher the material hardness, the more severe the pitting. Ensure the coating process selected is not detrimental to the function or durability of the component.