Hydrogen Embrittlement
Hydrogen  embrittlement  is  a major cause of fastener failure.  Prevailing thought is 
that  steels  with  Rockwell  hardness  above  C30 are vulnerable.  The phenomenon  
is  well-known  although  the  precise  mechanism has eluded extensive research.  A 
number  of  proposed  mechanisms  have  been  proposed,  and  most have at least 
some  merit.  Current  thinking  is that the susceptibility to hydrogen embrittlement is 
related  directly  to  the  trap population.  Generally,  hydrogen  embrittlement can be 
described   as    absorption   and   adsorption   of    hydrogen   promoting   enhanced 
decohesion  of  the  steel,  primarily  as  an  intergranular  phenomenon.                 
Electroplating  is  a  major  cause  of  hydrogen  embrittlement.  Some  hydrogen  is 
generated  during  the  cleaning  and  pickling cycles,  but by far the most significant  
source  is  cathodic  inefficiency,  which  is  followed  by  sealing  the hydrogen in the 
parts.   Baking  is  often performed on high strength parts to reduce this risk,  and the 
ASTM,  in 1994,  issued a specification for baking cycles,  as shown below.   For  the  
production   plater,    having   to   remove   the   parts   from   the   production  line  to 
bake  -  followed  by  a  separate  chromating  process -   is   a  laborious  process.  
              
HydroZinc3-2.JPG (33961 bytes)
      
Mechanically  deposited  zinc  coatings  consist  of  small  platelets formed from the 
mechanical  action  of  glass  beads  on fine (3 to 7 microns) zinc dust particles. The 
platelets  thus  formed  are "cold-welded"   to  the  substrate  and  to each other.  The  
porous  (approximately  80%  dense) deposit can therefore effuse the hydrogen that  
would  otherwise  produce  hydrogen  embrittlement.      
How much Baking Do Electroplated Parts need?                      
(ASTM B 850-94)      

 

  Tensile
  Strength
  (MPa)

  1700 - 1800
  1600 - 1700
  1500 - 1600
  1400 - 1500
  1300 - 1400
  1200 - 1300
  1100 - 1200
  1000 - 1100

Tensile
Strength
(000 psi)

247 -261
232 -247
218 - 232
203 - 218
189 - 203
174 - 189
160 - 174
145 - 160

Rockwell
Hardness
HRc

49 - 51
47 - 49
45 - 47
43 - 45
39 - 43
36 - 39
33 - 36
31 - 33

  Post-Plate Bake
  (@1900 - 2200 C.
  or  3740 - 4280 F.)

        22+
        20+
        18+
        16+
        14+
        12+
        10+
        8+

Per ASTM B 850-94 "For Steels of actual tensile strenth below
  1000 MPa, Heat treatment after plating is not essential."

 

For   Nearly  fifty   years  mechanical  Plating  has  been  accepted  as  a  means  of
eliminating  hydrogen embrittlement. Today,  many  specifications  reflect industry's
confidence  in  this  unique  process. While  it  is  true  that  mechanical plating uses
inhibited  acids  which  generate  less  hydrogen,  PS&T  believes  that  mechanical
platinng  as  a  process is inherently free from hydrogen embrittlement  because the 
deposit is porous (as are phosphate coatings), allowing hydrogen to escape through
the  coating;  in  electroplating,  by  way  of  contrast,  hydrogen  is  sealed in the part.

        
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