Avoid pitting corrosion: too high chloride concentration during anodizing

If the concentration of chlorides in certain chemical pretreatment steps is too high, this will lead to pitting corrosion on the components. Therefore, you should pay attention to the correct concentration in the pretreatment. In this article you will learn which values are relevant.

Too high chloride concentrations during anodizing

Chlorides are corrosion accelerators and should also be considered in the chemical pretreatment during anodizing. In the sulfuric acid anodizing bath, chloride catalyses the dissolution of aluminium and, in conjunction with corresponding current densities, promotes corrosion on the material. For this reason, the Qualanod quality association recommends maintaining a maximum chloride content of 100 mg/L in the anodizing bath, which corresponds to a quantity of 1 kg chloride dissolved in 10,000 liters of anodizing bath.

How can chloride corrosion be detected on the component?

When the limit value is exceeded, pitting corrosion occurs on the component, which can vary in intensity. Beginning with a diameter of approx 0.5 -2 mm, irregular depressions appear with partially frayed edges. The holes, which are often quite deep, appear black on the surface. The distribution is obviously random over the entire surface of the material, but it is often more prominent in the peripheral area of the component. If this type of corrosion occurs on the anodized material, it is of course no longer saleable.

How does the chloride get into the anodizing bath?

1. In regions with a high level of salt in the groundwater, there is a risk if correspondingly chloride-contaminated city water or well water is used. Here, a water analysis provides information on the current chloride load.
2. The use of an unsuitable sulfuric acid. Even though the review of sulfuric acid grades in Germany in 2018 (see leaflet A05, VOA -Verband für die Oberflächenveredelung von Aluminium ) generally showed chloride contents of less than 10 ppm, this possible source of error should not be completely disregarded in today’s global supply chains.
3. Human error. Human error can occur if the anodizing bath is accidentally adjusted with hydrochloric acid instead of the necessary sulfuric acid.

What steps need to be taken?

To eliminate a critical chloride content in the anodizing bath, the only option is usually to replace the corresponding bath solution. Theoretically, chlorides can also be precipitated in the form of poorly soluble silver salts, but this measure is ruled out for financial reasons. If necessary, mixed water or demineralized water can be used for the new preparation.