Anodizing defects: acid and alkaline corrosion

 

at aluminum anodizing, including – aluminum profiles, frequent defects include such, caused by, anyway, is one of the many types of aluminum corrosion. This occurs at various stages of the manufacturing process., starting from their improper storage and injection by improper packaging and transportation of anodized aluminum profiles.

Fingerprint corrosion

English term:

Fingerprint corrosion

defect Source

Inside- and inter-workshop movements.

Defect manifestation:

Travlenye

Details of the defect

Shallow, open ulcers, manifested by etching and anodizing of alloy profiles 6063 (drawing 1), although this defect may occur on other alloys. Ulcers form shapes, finger-like, which are not clearly visible before profile etching.

The cause of this defect is the local dissolution of the MgO / Al film2O3 on alloy profiles 6063 acid discharge from the hands of staff, worked with profiles.

korrozoya-otpechatkov-palcevFigure 1 – Fingerprint corrosion

Preventing defect

  • This defect does not occur., if you use clean cotton gloves when handling profiles.

Acid corrosion

English term

Acid corrosion

defect Source

Storage

The manifestation of the defect

Pickling - Anodizing - Staining - Filling

Details of the defect

Randomly located, often isolated, large, deep ulcers:

  • If corrosion is detected before etching and anodizing, there must be corrosion products in the ulcers.
  • If material was etched and anodized before inspection, then the ulcers will be clean and have an anode film.

The figure 2 corrosion is shown on an anodized and painted alloy profile 6063, resulting from exposure to acid contamination profile. In this case, the anode solution interacted with the colorant at the stage of staining or hot compaction. The figure 3 shows a cross section through a pitting corrosion (x200), caused by acid corrosion (intergranular corrosion).

kislotnaya-korroziyaFigure 2 – Acid corrosion

kislotnaya-korroziya-2Figure 3 – Acid corrosion – cross section (enlarged)

Preventing defect

  • The best way to deal with this type of corrosion – improvement of conditions for handling profiles and their storage.

Alkaline corrosion

English term

Alkali corrosion

defect Source

Storage

The manifestation of the defect

Travlenye - Anodyrovanye

Details of the defect

The figure 4 cross section through ulcer is shown, caused by alkaline corrosion. The sample was not etched and anodized., however, corrosion has developed very deeply and the ulcer has a very rounded appearance without intergranular corrosion.

The most common causes of alkaline corrosion are lime or cement dust., as well as moisture smudges. Alkaline pickling leaks or accidental contact with alkaline chemicals, used for adjusting alkaline solutions are other common causes of alkaline corrosion.

shchelochnaya-korroziya +Figure 4 – Alkaline corrosion (increased)

Preventing defect

Measures must be taken to eliminate the above causes of alkaline corrosion.. As with acid corrosion, if alkaline corrosion has already occurred, then it develops very deeply, so redoing a damaged profile is no longer possible.

Source: Barry R. Ellard, Aluminum Extrusion Technology, 2000