Sacrificial Corrosion Resistance of Galvanizing
An electroplated film has many pinholes, from which corrosion is considered to occur.
Galvanizing has the property of being sacrificed to electrochemically prevent corrosion of the steel.
In other words, the pinhole part exposed to the corrosive atmosphere causes white rust on the galvanized material but prevents corrosion of the steel.
Similarly, there is little worry that the steel on the cut surface of the wire will corrode first.
Corrosion Resistance of Nickel plating and Tin plating
In the case of nickel plating and tin plating, the pinhole part exposed to the corrosive atmosphere electrochemically corrodes the steel before the plated part, causing red rust to be generated.
Furthermore, on the wire-cutting surface, red rust is generated on the steel before nickel plating or tin plating.
White Rust on the Zinc Plated Surface
Since our galvanized steel wire is made of pure galvanized steel, exposing the wire to the corrosive atmosphere with moisture causes white rust on the galvanized surface.
The white rust is having dark gray appearance without metallic color or gloss peculiar to zinc.
This white rust prevents red rust from being generated on the surface steel while it remains.
Measures to Prevent White Rust
A method of performing chromate film treatment after plating has been widely used to prevent white rust on galvanized surface.
However, in the current situation, chromate treatment, using hexavalent chromium or trivalent chromium solution, cannot be performed from the viewpoint of focusing on environmental measures.
Under those circumstances, as a method to prevent white rust from the environmental viewpoint, there is a method of setting the heat treatment temperature to 300–320℃ after spring-forming process to change the galvanized film to zinc and iron alloy plated film by thermal diffusion.
This method can control the amount of white rust in the corrosion process to an extent that it is hardly noticed and dramatically improve corrosion resistance.
However, in this case, the spring surface tarnishes to a dull black color.
Salt Spray Test
This is a corrosion test method standardized by the Japan Industrial Standard (JIS) Z 2371, used to check corrosion resistance of metal and plated materials.
In this test method, after salt water with a concentration of 5% is sprayed for eight hours at a constant temperature of 35℃, the material is left unsprayed for 16 hours. This process is considered as one cycle (24 hours), judging the percentage of the red rust on the surface steel in the surface area of the material.
In fact, it is very difficult to use the test results to estimate the period that elapses until corrosion naturally occurs indoors or outdoors because this is a seriously extreme and condensed corrosion test method.
This should be only regarded as a testing method of comparing several kinds of materials.