The Effects of Salt Test Chamber and Salt Spray on Corrosion Testing

Salt is a ubiquitous compound, which not only exists in the ocean, atmosphere, land surface, lakes and rivers, but also causes machinery or electronic products to be exposed. However, the impact of salt fog environment should not be underestimated, as it can accelerate the corrosion of metals, leading to the damage of electrical and electronic products. In order to investigate the corrosion resistance of related products, Salt Spray test has become an important method. It can be seen that taking more full protection measures for the corrosion resistance of electronic products in salt fog environments is necessary.

Due to the presence of salt fog in the ocean, atmosphere, land surface, lakes and rivers, it is inevitable for electrical and electronic products to be exposed in salt fog environments. The influence of salt fog is only inferior to temperature, vibration, humidity, and dust environment, which can promote the corrosion of metals, constitute electrolyte and accelerate electrochemical corrosion, thereby resulting in the situations of corrosion damage of components or fasteners, blocked, failure, open loop or short circuit of mechanical components and components’ moving parts. In order to investigate the anti-corrosion ability of products in salt fog environment, Salt Test Chamber has become necessary. Therefore, it is of utmost importance to employ salt spray test to assess the anti-corrosive capability of electrical products in the corrosive environment.

For electronic products, the corrosion damage in the salt fog environment can mainly be attributed to the conductivity of salt solution and the increase of resistance caused by salt fog corrosion. This will inevitably lead to an increase in resistance and a decrease in voltage, which will seriously affect the electric performance of the product and cause degradation. Therefore, it is necessary to take appropriate protective measures, such as special coatings such as metal coatings, oxide films, etc., which can not only restrain metal corrosion, but also make the product more weather-resistant. In addition, attention should also be paid to structural design, reducing the surface area of components and contacts, and adding appropriate sealing structures at the contact points of components, in order to further enhance the product’s ability to resist corrosion.

1. Salt Spray Test Categories
Salt spray testing is an effective method for assessing the corrosion resistance of metallic materials and their coatings to salt spray corrosion, generally classified into neutral salt spray test (NSS), acetic acid Salt Spray (AASS) and copper accelerated acetic acid salt spray (CASS). Neutral salt spray test is the most widely used, mainly used to detect the corrosion resistance of metallic materials and their metal or inorganic non-metallic coatings. Acetic acid salt spray and copper accelerated acetic acid salt spray are mainly used to detect the corrosion resistance of metal coatings, not for organic coatings.

Salt Test Chamber is used to evaluate the ability of product surface to resist salt spray corrosion, and the test results reflect the condition of surface damage, such as blistering, rusting, adhesion decline and spreading of corrosion in scratch area. The comprehensive evaluation criterion for test results is not yet universal. It is clearly recommended in GB/T 1766-2008 Paint and Clear Coating Degradation Ratings and GB/T 6461-2002 Ratings of Test Samples and Specimens after Corrosion Test on Metal Base with Metal or Other Inorganic Coating.

2. Surfaces Corrosion Textures Generated by Salt Spray
Normal surface corrosion is a common type of material erosion, which is mostly removed from the contact surface of material uniformly and the general types are flake and face erosion. Pitting corrosion (hole erosion) is corrosion of limited localities, which penetrates into the material and forms holes or depressions with deeper depth than the diameter, and there is no metal removal on the surface outside the corrosion area. Crevice corrosion mainly occurs in narrow crevices and due to the difference of concentration in corrosion medium, it will cause potential difference between both ends of the crevice, which leads to aggravation of corrosion in the areas with poor ventilation. Dezincification is a selective dissolution, which leaves a porous cupric structure of zinc removed from brass, and de-nickelization and de-aluminization processes are similar. Rust is formed on iron and steel by the corrosion products of ferrous oxide and hydroxide, which also causes dull surface and reduction of luster.

Salt Spray corrosion is electrochemical, and its main mechanism is anodic activation that metal atoms leave the lattice, the potential difference leads to oxidation reaction on anode, and the anodic dissolution of metal obtains certain amount of electrons and causes the metal ions to dissolve into the electrolyte; while the results of cathodic reaction are sodium chloride dissolution and reaction with metal ions and hydroxyl ions, eventually forming metal corrosion products. Three elements are water, oxygen and ions, the coatings can slow down its pass through to different extents, only when the salt concentration in water exceeds 0.4mol/L, sodium and chlorine ions can penetrate through the coating and play the role of corrosion, and its specific corrosion ability depends on the metal potential series and pH of medium.

3. Metal Potential Chess of Metals
The metal potential chess of metal is the reference relationship between metal and corrosion polarization when no external voltage is applied. According to the difference in potential, metal can be divided into very noble metals (potential less than -0.5V), noble metals (potential from -0.5V to 0V), semi-noble metals (potential from 0 to +0.7V) and noble metal (potential greater than +0.7V).

Very noble metals such as Na, Mg, Be, Al, Ti and Fe will be corroded in neutral water solution even without oxygen. Noble metals (such as Cd, Co, Ni, Sn and Pb) can be corroded in neutral water solution both in presence or absence of oxygen and release hydrogen in acid. Semi-noble metals (such as Cu, Hg and Ag) can only be corroded in oxygen-containing environment in various solution, and noble metals (such as Pd, Pt and Au) are usually stable.

In conclusion, the presence of oxygen helps the corrosion of metal, higher potential metals are more stable, and lower potential metals may be corroded without oxygen environment. Therefore, to ensure the stability of metal, engineers need to select and use metal with higher potential on the basis of considering the correct oxygen partial pressure condition and use different types of Salt Spray tests for different products.

Tags：YWX/Q-010