How do I know if the coating or plating I am stocking will meet a specific customer’s requirements?
Question:
How do I know if the coating or plating I am stocking will meet a specific customer’s requirements?
Answer:
Fastener descriptions for platings and coatings can be quite confusing. It is often difficult to determine if the description listed is for a standard plating or if a special coating is required. Platings and coatings are mainly applied to fasteners to provide corrosion protection. The amount of corrosion protection that the coating provides is typically measured in what is called a “salt spray test”. Coatings will have minimum hours to rust requirement when tested in a salt spray test. Callouts for corrosion protection can be total hours to red rust or for zinc-based coatings the callout can include hours to white corrosions as well as to red rust.
Listed below are the five most commonly available platings or finishes that fasteners are stocked in and the plating spec they are commonly referenced.
Plating/Finish Description
Plain -
No additional coating is added to the surface of the fastener after manufacturer and/or heat treatment.
- The product will typically have a light soluble oil applied to prevent rusting during storage.
- Non-heat treated fasteners will be dull gray in appearance, heat treated or hot formed fasteners will have a splotchy black appearance.
- Very limited or no salt spray hours are provided.
- No plating spec referenced
Phos and Oil –
A zinc phosphate compound and oil is applied to the fasteners (manganese or iron phosphates are also available).
- Coating appearance is a dull gray to black (it is difficult to distinguish Plain heat treated fasteners from a Phos and Oil coated one.
- Commercial phos and oil coating will typically provide 24 hrs to red rust salt spray.
- Common industry plating spec called out for Phos and Oil is ASTM F1137 Grade OC.
Zinc Plating –
When a customer calls out a “zinc” plating, the standard plating that is supplied is a two layer plating that has a zinc base and a chromate topcoat (this type of plating is also called Zinc/Clear or Zinc with Clear Chromate.
- The base layer is metallic zinc that has a thickness of .0002” (also called 5 microns) min.
The second layer is a clear chromate film (the chromate does not add any significant thickness to the overall plating thickness).
- Coating appearance is a bright silver.
- Many customers will require that the clear chromate is:
RoHS compliant
Trivalent
Cr3+
Non-Cr6
All of the above callouts mean the same thing.
- Zinc platings will typically provide corrosion protection that meets 24 hrs to white corrosion and 72 hrs to red rust.
- Common industry plating spec for Zinc plating is ASTM F1941 Fe/Zn 5AN (formerly 5AT).
Zinc/Yellow
- When “yellow” is added to the zinc plating description, a yellow chromate is applied over the zinc base coat.
- Coating appearance is Yellow to Bronze (yellow chromates can have an “iridescent” appearance that mixes a slight green and violet color in with the yellow).
- A yellow chromate provides additional corrosion protection over a clear chromate and is expected to provide 72 hrs to white corrosion and 96 hrs to red rust.
- Standard yellow chromates are not RoHS compliant and will contain hexavalent chromates (Cr6). RoHS complaint zinc yellows are available but must be ordered special.
- Common industry plating spec called out for Zn/Y is ASTM F1941 Fe/Zn 5C or B633 Type II SC1.
HDG- Hot Dip Galvanizing
Hot dip galvanized is a common coating used with structural fasteners (ASTM A307or ASTM A325).
- HDG is a heavy layer of zinc that also can have a clear chromate top coat.
- The coating thickness is typically specified by the weight of coating and will have a thickness that is up to ten times heavier than a zinc plating.
- Coating appearance is a dull gray (not shiny like a zinc plating).
- Common industry spec callout for HDG is ASTM A153 Class C or D.
If you have questions on a plating callout, just contact your Earnest representative and we can help you identify the type of coating you need.