Corrosion Resistance

For increased corrosion resistance and for customers' specific requirements, a range of standard finishes is available. The matrix below compares the various features of these finishes and can be used as a guide to selecting the most appropriate surface finish for an application.

Note

European Directive 2000/53/EC Vehicle End of Life (ELV) Legislation states that hexavalent chromium (Cr6+), lead, mercury, and cadmium are substances banned from vehicles marketed after 1 July 2007. Our fasteners do not contain cadmium or mercury.

However, yellow passivation treatment of zinc and zinc alloy plating contains hexavalent chromium (Cr6+) and some hot tin dipped coatings contain lead.

In conjunction with metal finishing companies, the automotive industry, and the electronics industry, we have developed finishes that are free from both Cr6+ and lead.

	Corrosion Conditions	Galvanic Corrosion	Hexavalent Chromium	Electrical Conductivity	Temperature Resistance	Appearance	Availability	Cost
Zinc plate + yellow passivation		Refer to Chart	Yes	Conductive	Moderate/ Low	Iridescent Yellow	Moderate/ Good	Low
Zinc/nickel plate + yellow passivation		Refer to Chart	Yes	Conductive	Good	Iridescent Yellow	Moderate/ Good	High
Zinc plate + clear passivation		Refer to Chart	No	Conductive	Moderate	Clear/ Metallic Blue Tinge	Good	Low
Zinc plate + clear passivation + sealer		Refer to Chart	No	Conductive	Moderate	Clear/ Metallic Blue Tinge	Good	Moderate
Zinc/nickel plate + clear passivation		Refer to Chart	No	Conductive	Good	Clear/ Metallic Blue Tinge	Good	High
Zinc/nickel plate + black passivation		Refer to Chart	No	Conductive	Good	Black	Good	High
Zinc plate + Deltaseal		None	No	Non Conductive	Moderate	Black Silver	Good	Moderate
Zinc/nickel plate + Deltaseal		None	No	Non Conductive	Good	Black Silver	Good	High
Phosphate and Deltaseal		None	No	Non Conductive	Moderate	Black Silver	Good	Moderate
Zinc/nickel plate + KTL		None	No	Non Conductive	Good	Black	Moderate	High

	Light: Typically indoors and warm dry atmospheres.
	Moderate: Typically indoors where moisture can occur. Outdoors in mild corrosive conditions.
	Severe: Typically outdoors in moderate corrosion conditions; for example, light industrial and possibly external vehicle applications.
	Very severe: Typically outdoors in severe corrosive conditions; for example, marine, industrial, or vehicle engine compartment or under body.

Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals are in close contact with an electrolyte, a medium through which an electrical current can flow. The presence of water as moisture can act as an electrolyte. The rate of corrosion depends upon the differences in electrical potential, or the anodic-cathodic relationship, of the metals in the joint as defined by the Galvanic Series of Metals & Alloys. (See below).

A highly anodic metal in contact with a highly cathodic metal will corrode much more quickly than two highly cathodic metals or when the metals are closer together in the galvanic series.

When corrosion occurs it is the anodic metal most likely to corrode and the cathodic metal least likely to corrode.

To reduce the likelihood of galvanic corrosion in a fastened joint, it is recommended that the designer choose metals that are grouped together in the galvanic series chart. If that is not possible, other recommendations are:

• Select metals which are as close together in the chart as possible.
• Provide a barrier between the metals, such as paint, non-metallic washer, gaskets, or jointing compound.
• Design the fastener as the cathode so that the cathodic area is as small as possible to the anode area.     
• Use a metallic finish on the fastener that is close on the chart to the mating metal.
• Use a non-conductive and inert finish on the fastener.

Galvanic Series

Anodic End (Most Likely to Corrode)

• Magnesium and its alloys
• Zinc and zinc plating on steel
• Aluminium Alloys
• Cadmium
• Mild Steel & Iron
• Cast Iron
• 13% Chromium stainless steel (active)
• 18-8 Austenitic stainless steels (active)
• Tin-Lead, Lead, and Tin
• Nickel (active)
• Brasses
• Copper
• Bronzes
• Cupronickel Alloys
• Silver Solder
• Nickel (passive)
• Iron-chromium alloys (passive condition)
• 18-8 austenitic stainless steels (passive condition)
• Silver
• Titanium
• Gold and other precious metals

Cathode (Least Likely to Corrode)