Surface finish overview

 

The standard finish for many steel fasteners is electroplated zinc with a clear RoHS compliant (trivalent) chromate passivation. This finish offers basic corrosion protection and is inexpensive. The major disadvantage of any electroplating process is that it can cause hydrogen embrittlement in fasteners with a tensile strength greater than 320 HV.

As standard, all fasteners that are case hardened and all fasteners with a property class of 8.8 or higher are heat treated after electroplating to help reduce the risk of hydrogen embrittlement. This process is called de-embrittlement, but it does not guarantee that the risk of hydrogen embrittlement is removed completely. If any risk of hydrogen embrittlement is unacceptable than a non-electroplated finish should be considered.

There are many finishes available today offering various degrees of protection and cosmetic finish. Some of the most common ones are outlined below. For more information on any of the finishes including corrosion protection data where available, see the full article.

 

 

Electroplated zinc

Product Nuts and Screws < Grade 8 or 8.8  <320 Hv <1000Nmm2 Nuts and Screws  >= Grade 8 or 8.8  >320 Hv >1000Nmm2 Case hardened fasteners Washers <= 320 Hv Washers >= 320 Hv
Size M1 to M5 M6 and above M1 to M5 M6 and above M1 to M5 M6 and above All sizes All sizes
Recommended YES YES YES YES YES YES YES YES
De-embrittle? NO NO YES YES YES YES NO YES
Standard coating 3 to 5 microns 5 to 12 microns 3 to 5 microns 5 to 12 microns 3 to 5 microns 5 to 12 microns 3 to 12 microns 3 to 12 microns
to white rust zinc & clear 2 to 6 hours 6 hours 2 to 6 hours 6 hours 2 to 6 hours 6 hours 2 to 6 hours 2 to 6 hours
to red rust zinc & clear 12 to 24 hours 24 to 72 hours 12 to 24 hours 24 to 72 hours 12 to 24 hours 24 to 72 hours 12 to 72 hours 12 to 72 hours
Chromate treatment Clear, bleached, black, iridescent, opaque
 

Notes

Corrosion resistance (Salt Spray to BS 7371-3: 1993) figures shown are based on zinc & clear.

Other types of chromate treatment can increase corrosion resistance. See full electroplated zinc page for details.

 

 

Organic

Product Nuts and Screws < Grade 8 or 8.8  <320 Hv <1000Nmm2 Nuts and Screws  >= Grade 8 or 8.8  >320 Hv >1000Nmm2 Case hardened fasteners Washers <= 320 Hv Washers >= 320 Hv
Size M1 to M5 M6 and above M1 to M5 M6 and above M1 to M5 M6 and above All sizes All sizes
Recommended NO YES NO YES NO YES YES YES
De-embrittle? N/A NO N/A NO N/A NO NO NO
Standard coating                
to white rust   72 to 240 hours   72 to 240 hours   72 to 240 hours 72 to 240 hours 72 to 240 hours
to red rust                
Colour To customer specification
 

 

Zinc flake

Product Nuts and Screws < Grade 8 or 8.8  <320 Hv <1000Nmm2 Nuts and Screws  >= Grade 8 or 8.8  >320 Hv >1000Nmm2 Case hardened fasteners Washers <= 320 Hv Washers >= 320 Hv
Size M1 to M5 M6 and above M1 to M5 M6 and above M1 to M5 M6 and above All sizes All sizes
Recommended NO YES NO YES NO YES YES YES
De-embrittle? N/A NO N/A NO N/A NO NO NO
Standard coating                
to white rust   240 to 480 hours   240 to 480 hours   240 to 480 hours 240 to 480 hours 240 to 480 hours
to red rust                
Colour Generally silver or black but can be other colours