What active elements are affected by spot corrosion of LED aluminum profiles

1 Acceleration effect of Zn element

The zinc solid dissolved in the LED aluminum profiles accelerates grain corrosion in a "dissolution-redeposition" manner. The zinc or iron deposited on the alloy(LED aluminum profiles) surface as well as cathodic particles such as high-potential desolvation products FeSiAl and free silicon can act as effective cathodes. It accelerates the reduction process of dissolved oxygen and promotes the continuous expansion and deepening of corrosion.

During alkali cleaning, the Zn element dissolves in the alkali solution in the form of Zn(OH)42- and Zn(OH)-3 along with the dissolution of Al. And because the potential of Zn (-0.76V) is more positive than the potential of Al (-1.67V), when the concentration of Zn ions in the alkali solution increases to a certain value, Zn will selectively deposit on the residues in the corrosion pits. On the material, there will be an abnormal phenomenon of high Zn element. On the other hand, due to the large potential difference between Zn and Al, the corrosion current in the microbattery is very large, and the cathodic particle Fe and Si-poor areas (basically pure aluminum) dissolve quickly. This corrosion ultimately manifests as Spot corrosion.

2 Activation of Cl-

As an external factor, Cl- is very sensitive to pitting corrosion and has the effect of inducing and aggravating pitting corrosion. The research results found that Cl- in the degreasing acid will be adsorbed at the defects of the passivation film, and penetrate the passivation film to be adsorbed on the substrate. The aluminum element here dissolves quickly due to activation, so the passivation film is destroyed, forming a galvanic cell structure. Under the action of the acidic medium, the local corrosion current is large. At this time, Cl- and the dissolved A13+ undergo the following complexation Reaction: Al3++Cl-+H2O→AlOHCl++H+, which further strengthens the acidity of the solution and worsens the corrosion conditions. When the Cl- concentration increases, the complexing reaction proceeds to the right, and the active points on the passivation film will be greatly increased, which will be preferentially dissolved during the subsequent alkali cleaning process, resulting in more serious spot corrosion.

3 Promoting effect of pH value

When the pH value of the washing water is less than 2 or greater than 4, spot corrosion rarely occurs. When the grains turn dark in color from gray to black, the pH value in the washing tank plays a certain role in promoting it.

When the pH in the washing water is >4, the passivation film formed on the surface of the LED aluminum profile is relatively complete and dense, and the adsorption, activation, and destructive effects of H+ and Cl- are greatly weakened, so there is little or no corrosion on the profile; when the pH is <2, The surface of the aluminum profile is in an active dissolution state and no passivation film is formed, so there will be no spot corrosion.

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