Coating Designation and Corrosion Resistance for Electroplated Zinc

according to BS 7371 Pt 3 :1993 

DesignationNominal Coating Thickness1st Appearance of White corrosion product (Hrs)1st Appearance of rust (Hrs)
Zn3 Clear3µm212
Zn3 Bleached812
Zn3 Iridesent1624
Zn3 Opaque2424
DesignationNominal Coating Thickness1st Appearance of White corrosion product (Hrs)1st Appearance of rust (Hrs)
Zn5 Clear5µm624
Zn5 Bleached1236
Zn5 Black12 
Zn5 Iridescent4872
Zn5 Opaque7296
DesignationNominal Coating Thickness1st Appearance of White corrosion product (Hrs)1st Appearance of rust (Hrs)
Zn8 Clear8µm648
Zn8 Bleached2472
Zn8 Black2472
Zn8 Iridescent72120
Zn8 Opaque96144
DesignationNominal Coating Thickness1st Appearance of White corrosion product (Hrs)1st Appearance of rust (Hrs)
Zn12 Clear12µm672
Zn12 Bleached2496
Zn12 Black2496
Zn12 Iridescent72144
Zn12 Opaque96168
Chromate Treatment 
TypeTypical AppearanceCorrosion Protection
ClearTransparent clear sometimes bluish tinge.Slight,for example against staining during handling or against high humidity in mildly corrosive conditions
BleachedTransparent with slight iridesence
BlackBlackwith slight iridesence
IridesentYellow IridesenceConsiderable, including organic vapours
OpaqueOlive green, shading to brown or bronze

Note: Due to the fact that fastening applications differ greatly, the above information is for guidance only and is correct to the best of our knowledge. The customer must satisfy themselves with the performance of the fastener and validity of the data. TR Fastenings will not be held responsible for any failure that may occur from the use of this information.