Anodizing defects: “tinsel”, “prižog” and “loss of contact”

See. also Classification of defects in anodizing aluminum

Electroplating (glitter)

English term

Spangle

Causes

  • alkaline solution composition
  • high zinc content in aluminum alloy

Manifestation

alkaline etching

Description

Surface heterogeneity aluminum profile after etching, from a "grainy" or "galvanized" surface to a surface with selective grain etching (Figure 1).

Figure 1 – Extruded aluminum alloy surface 6063
defective “galvanization” (“tinsel”).
Visible steps between grains high 5 μm (x 2000) [2]

The cause of this defect is the presence of zinc as an impurity in the aluminum alloy or alkaline pickling solution.. This defect can also occur during etching of aluminum profiles in an alkaline solution of high concentration or at elevated temperature..

Hollow aluminum profiles are more prone to this defect., than solid profiles, because, what do they have, usually, larger grain size, and also because, that when etching the inner surface of the profile, an increased temperature of the solution occurs due to the limited circulation of the etching solution in its cavity.

Anodizing defect “galvanization”(“tinsel”) most often occurs:

  • When using fresh pickling solutions, for example, after solution regeneration
  • If aluminum alloy has a high zinc content
  • With an increased content of free zinc in an alkaline solution.

Prevention

  • Limitation of zinc in aluminum alloys, for example, alloys AD31, 6060 and 6063, below 0,03 % (drawing 2) [2]
  • Maintenance of the content of free zinc in alkaline etching solution is not higher 50 ppm (0,0050 %) [2].

Figure 2 – Slight anodizing defect “galvanization” (“tinsel”) on extruded aluminum alloy profiles 6063
with zinc 0,04 % [2]

This defect does not occur there., where "long-playing" alkaline solutions are used (“E6 solutions”), since they contain sulfides and nitrates, that bind free zinc. For this reason, many “long-playing” alkaline pickling solutions turn very black.

Anode burn

English term

Anodizing burn.

reason

Anodizing

Manifestation

Anodizing

Description

Soft, powdered anode coating on aluminum profiles or, in more serious cases, complete dissolution of the coating with the effect of electro-polishing of the aluminum surface at the defect site. Anodic burn occurs with excessive current density and inadequate mixing of the anode solution (Figure 2).

defekt-anodirovaniya-prizog

Figure 3 [1]

Prevention

This defect does not occur with the correct anodizing technology:

  • correct hitching of profiles to ensure uniform distribution of current,
  • good movement of the electrolyte through the hitch, and
  • correct current density ratio, composition of the anode solution and temperature.

Loss of electrical contact

English term

Loss of electrical contact

reason

Hanging - Anodizing

Manifestation

Anodizing

Description

Light- or dark gray surface of aluminum profiles, on which traces of a matrix or strip are visible and which may have a rainbow tint.

To demonstrate this difference in surface appearance, on the image 4.8 aluminum profile shown, half of which is etched and anodized to normal anode thickness (left), and the other half is left in the anode solution without connecting the current (right).

defekt-anodirovaniya-poterya-kontakta

Figure 4 [1]

Sample surface, which was not properly anodized, shows rainbow colors, characteristic of a very thin anode coating. This effect is less noticeable., if the electrical contact was lost in the middle of the anodizing process - in this case, the defect is detected only by measuring the actual thickness of the anode coating.

Prevention

Aluminum profile can be machined, if possible, and re-anode. With well-developed technology for hanging aluminum profiles, this problem is minimized.

Sources:
1. Bаrry R. Ellard, Aluminum Extrusion Technology, 2000
2. T. Short, The Identification and Prevention of Defects on Anodizing Aluminium Parts, 2003