APPLICATION TECHNOLOGY
Surface finishing for fasteners in a marine environment
By Alan Gardner, global marketing manager, MacDermid Industrial Solutions
Cadmium, combined with a hexavalent chromate conversion coating, has been the preferred choice for protecting fasteners in a marine environment for many years now. This coating provides long-term sacrificial protection, without the excessive growth of white corrosion products.
T
he features of cadmium plating in this application can be summarised as: • Self-healing (sacrificial) on ferrous substrates. • More than 500 hours to base metal corrosion (neutral salt spray testing).
• Minimal corrosion products. • Soft and ductile – acts as a thread lubricant. • Compatibility with aluminium.
However, engineers are searching for alternatives to these
coatings which can meet the above criteria due to the toxicity of cadmium and hexavalent chromium compounds.
Alternative to cadmium and hexavalent chromium Alternatives to cadmium and hexavalent chromium are based
on zinc with a trivalent chromium passivate. Zinc can be readily alloyed with metals such as cobalt, iron and nickel. Of these, zinc-nickel (containing some 12% - 15% nickel) offers the best alternative to cadmium. Table 1 illustrates this.
Table 1: Performance criteria for zinc and zinc alloy finishes
Criteria
Sacrificial protection
> 500 hours to base corrosion
Minimal corrosion products
Lubricated thread
Zinc Zinc-iron Zinc-cobalt Zinc-nickel Yes
Yes No Yes
Compatibility with aluminium No No
No No Yes Yes Yes No With appropriate lubricant
Sacrificial protection Corrosion is an electrochemical process in which, for example,
steel oxidises (rusts). Oxidation occurs as a result of the chemical reaction between steel and oxygen, in the presence of water. Sacrificial protection prevents this reaction by galvanically protecting the steel with another metal in contact with the steel. When base steel is exposed, such as at a cut edge or scratch, the steel is cathodically protected by the metal. This occurs because metals such as cadmium and zinc/zinc alloys are more electronegative (more reactive) than steel in the galvanic series. See table 2:
Yes Yes Yes Yes Active (anodic)
Table 2: Galvanic series in seawater
Metal Noble (cathodic)
Cast iron Steel Duralumin Cadmium plating Aluminium (Commercial purity) Zinc
Magnesium
Resistance to base metal corrosion In marine environments, cadmium corrodes very slowly producing
minimal corrosion products. This helps to explain why cadmium, despite toxicity issues, is still used in applications where it is exposed to sodium chloride (salt). Zinc by comparison corrodes rapidly. This is explained by Hayashi (1991): “Zinc in the coating layer
changes to chloride at the initial stage of the corrosion followed at a later stage to zinc oxide. The substrate corrodes after the zinc coating loses its ability to sacrificially protect. By contrast zinc alloy coated steel sheets showed excellent corrosion resistance. This result was explained in terms of zinc alloy coatings being more durable against chlorine ion penetration into the coating layer than zinc coating”. For this reason zinc alloys are preferred when replacing
cadmium in chloride environments. For an equivalent thickness, normally 8µm on fasteners, zinc alloys match cadmium deposits in neutral salt spray tests. It is very important that deposit thicknesses do not exceed this otherwise dimensional tolerances of the fastener are compromised. For this reason, increasing the thickness of zinc is not a viable alternative.
Minimal corrosion products Plated zinc rapidly produces voluminous corrosion products
in the presence of sodium chloride. Zinc-nickel, like to cadmium, shows minimal corrosion products in this environment. In neutral salt spray testing, these corrosion products are referred to as white rust. Table 3 demonstrates the resistance to white rust of iridescent passivated coatings.
Table 3: Comparative values to 5% white rust for passivated coatings
Criteria Hours to white rust 132 Fastener + Fixing Magazine • Issue 76 July 2012
Zinc Cadmium Zinc-nickel > 72
> 96 > 150
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