Electroplated Coatings  
 
Details :
   
Standard
 

ISO : 4042
DIN : 267 Part 9
ANSI :

   
Scope and field of application
 

These technical conditions are in particular related to threaded fasteners (mainly bolts and nuts), but are also applicable to the whole range of mechanical fasteners.

 
Electroplated coatings

 

An electrolytically applied coating shall be defined as a protective metallic layer being deposited onto the surface of metal articles by immersing these parts in an aqueous solution through which an electrical current is passed.

 
   
Layer thickness, measuring point and measuring method
 

Because of the variations in layer thickness on electroplated surfaces on mechanical fasteners, the local layer thickness is measured at a given spot considered significant for the purpose of assessing the protection against corrosion.

 

  The layer thickness can be measured by:
- direct determination in accordance with DIN 50933
- the jet method is accordance with DIN 50951
- the coulometric method in accordance with DIN 50955
- microscopic determination in accordance with DIN 50950
In borderline cases the last method is governing.
   
Screw thread tolerances
 

The basis for the layer thickness of electroplated coatings is given by the tolerances for ISO-metric screw threads in accordance with DIN 13 resp. ISO-unified screw threads in accordance with ISO 5864 (ANSI B1.1) prior to the electroplating that means tolerance field g resp. 2A for bolts and screws and H resp. 2B for nuts.
The coating must not cause the zero line to be exceeded, so bolts and screws have to meet the go-gauge with tolerance field h resp 3A and a measurable layer thickness can only be applied to nuts on condition that the tolerance field H resp 2B is not being fully utilized down to the zero line.

   
Hydrogen embrittlement
  Due to the risk of hydrogen-induced delayed brittle fracture bolts and screws with a tensile strength Rm >= 1000 N/mm or a hardness >= 300 HV (F >= 98N) have to be baked on 200 100C as soon as possible but within 4 hours after the coating process. This is also mandatory for resilient (springy) fasteners with hardness >= 400 HV (F >= 98N)
   
Passivation by chromate treatment
 

This after-treatment has to be carried out in accordance with DIN 50941 and after baking. The protection against corrosion is considerably increased by chromatizing. Out of the different colours from bluish (white) to black , yellow passivation is preferred.

 
 
     

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