Influence of impurities on depth scattering during electrolytical tin colouring of aluminium

An experiment was conducted to investigate which impurities interfere with depth scattering during electrolytical tin colouring. The aim of the experiment was to determine the influence of individual foreign ions on the homogeneity of the deposited tin layer and the associated optical scattering properties.

Experimental procedure

For the experiment, EN AW 5005 metal strips were first chemically pretreated and anodised in the same way. Then 6 cells were filled with tin colouring bath and each was contaminated with different foreign ions that could potentially interfere with the deposition process. These included:

• • Approx. 1200 mg/L chloride

• • Approx. 300 mg/L nitrate

• • Approx. 1000 mg/L phosphate

• • Approx. 2000 mg/L oxalic acid

• • Approx. 2000 mg/L sodium and

• • Approx. 2000 mg/L aluminium

After electrolytical colouring of the metal strips in the respective cells, the samples were compared optically with regard to their depth scattering. The coloured metal strips are shown in the figure from left to right. The depth scattering results from the loss of colour from left to right. The more intense the colouring of the metal strips on the right-hand side, the better the depth scattering of the bath solution prepared. Of course, the effect can be more pronounced with higher concentrations of impurities.

Observation of the results

The test data show that the addition of specific impurities leads to different disturbances in the deposition process. The detailed observations are as follows:

• • Contamination by chloride:
    While there is only a moderate decrease in depth scattering, there is a complete loss of the entire oxide layer due to spalling (left).
    
    
  • Sodium contamination:
    Compared to the other contaminants, there is a significant loss of depth scattering at this concentration.
    
    
  • Nitrate contamination:
    The negative effect on the depth scatter of the dye bath is most evident here. Even 300 mg/L leads to a massive loss of colour.
    
    
  • Contamination by phosphate:
    In the experiment, the least influence on the depth scattering of the bath can be seen here.
    
    
  • Oxalic acid or aluminium
    Here there is a slight influence on the depth scattering of the bath, but this is less pronounced than with sodium.

Evaluation of the results

Nitrates interfere most strongly with the depth scattering of the electrolytical tin colouring bath. Even small quantities are sufficient to prevent correct colouring. Nitrates can, for example, enter the colouring bath over time via the transport of goods when using a nitric acid desmutting and thus gradually cause disturbances. The good news is that nitrate interference can be effectively eliminated by adding Correction solution 600. In most cases, the original colouring performance of the bath can be restored.