Deposition of electrolytic zinc alloys on steel
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle:
Alloyed zincs have been developed to meet changing automotive specifications for improved corrosion resistance, even under thermal stress in the engine compartment, control of the coefficient of friction and improved wear resistance.
History of zinc alloy coatings
The first commercial alloys appeared in the 1980s with the first co-deposits of Iron (0.4 to 0.6% Fe) or Nickel (4 to 8% Ni) in alkaline media, followed by Zinc-Cobalt (0.5 to 1.5% Co).
But the only true Zinc-Nickel alloy (12 to 15% Ni) in its gamma phase of the Zn-Ni binary diagram was first developed in an acidic ammonium chloride medium before being rapidly declined in an alkaline medium for more alloy stability at the end of 1993.Processes
The corrosion protection of steel is primarily due to the anodic potential difference between zinc (ESCE = approx. -980 mV) and steel (ESCE = approx. -400 mV). The steel is thus protected by cathodic protection as long as the zinc is not completely oxidized.
A co-deposit of iron or cobalt at a maximum of 1.5% does not modify this potential and Zn-Fe and Zn-Co are not intended to resist corrosion longer. On the other hand, they favor chromium passivation, which becomes thicker and more resistant.
Zn-Ni between 4 and 8% Ni offers an intermediate behavior since it is a mixture of pure zinc phase and gamma phase of Zn-Ni at 12-15% Ni.
Only the 12-15% Zn-Ni has a behavior quite close to Cadmium with an anodic potential closer to that of steel, i.e. ESCE = about -700 mV.
The rate of corrosion in contact with steel is thus reduced, especially since the corrosion mechanism involves a loss of zinc that ennobles (returns to a less anodic potential) the deposit as it corrodes. During the corrosion attack, the potential of Zn-Ni evolves slowly and approaches that of iron.
The selection of the best process therefore involves the choice of a thicker passivation with Zn-Fe or Zn-Co co-deposits or the choice of an intrinsically more protective alloy that does not require thick passivations of the Zn-Ni type.
Another significant advantage of 12-15% Zn-Ni in alkaline media is its ability to not embrittle hard steel up to bolt grade 12.9 by hydrogen. It is a thesis (El Hajjani, UFR of Besançon, 2007) which showed that the deposition of Zn-Ni 12-15% in alkaline medium is carried out initially by the deposit of a few nanometers of a layer of pure nickel. This small intermediate deposit of pure Nickel would serve to trap the hydrogen that naturally effuses to the surface through the micro-cracked Zn-Ni 12-15% deposit.
A new ammonium-free acidic Zn-Ni 12-15% process has many advantages in terms of appearance and deposition rate but cannot claim the same non-embrittling functions because the intermediate layer does not exist.
All of the above mentioned alloyed zincs are passivated with trivalent chromium and thus comply with the new ELV (2000/53) and RoHS (2002/95) Directives.
Composition of electrolytes
Zinc-cobalt acid :
TENSIONS IN G/L
COMPONENTS
25-40
Zinc
Cobalt
2-5
130-180
Total Chloride
200-250
Potassium Chloride
25
Boric acid
Zinc-nickel alkaline 4-8% :
TENSIONS IN G/L
COMPONENTS
7.5-10
Zinc
1.8-2
Nickel
100-120
Soda
Alkaline zinc-nickel 12-15% :
TENSIONS IN G/L
COMPONENTS
7-12
Zinc
1-2.5
Nickel
120
Soda
Zinc-nickel acid 12-15% :
TENSIONS IN G/L
COMPONENTS
30-40
Zinc
25-35
Nickel
150-230
Total Chloride
25
Boric acid
Implementation
Main equipment (furnace, reactor, line, machine...)
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