Common defects in anodic oxidation of aluminum alloys

Appearance defects are the main reason for the rework of the profile, which greatly increases the cost. This paper summarizes the main characteristics, causes and countermeasures of appearance defects of aluminum anodic oxide film. According to the appearance morphology, the surface defects of anodic oxidation can be divided into three categories:(1) stripe (band)-like Defects, (2) Spotted defects, (3) uneven (abnormal) surface . Since stripe (belt) Defects often result from casting and squeezing, or other mechanical damage, this article only introduces the latter two common defects.

1 Spotted defects
Material corrosion, bath contamination, alloy second phase precipitation or galvanic effect and other factors can lead to spot-like defects, respectively, described as follows:

1.1 Acid or alkali etching
Before Anodic oxidation, due to the aluminum splash acid or lye or by acid fog or alkali mist action and corrosion, so that the surface of local white spots. If the corrosion is more serious, the pitting is thicker and the coarse spot is formed. It is difficult for the naked eye to distinguish the cause of acid or alkali, but under the microscope to observe the cross-section of the pitting is easy to distinguish, such as the bottom of the garden shape and no intergranular corrosion signs, resulting from alkali corrosion, such as the bottom of irregular and with intergranular corrosion, pitting and deeper caused by acid corrosion (5).
This type of corrosion can also be caused by improper plant storage. Chemical polishing agent smoke or other acidic smoke, chlorine-containing organic degreasing agent is the source of acid erosion. The most common alkali leaching by mortar or cement ash, alkali lotion and other substances scattered and

Splash caused. Once the cause is determined, the problem can be solved by strengthening the management of the plant. 1.2  atmospheric corrosion
Aluminum profiles are exposed to humid air and sometimes have white spots, which are often arranged vertically along the direction of the mold trace. Atmospheric corrosion is generally not as serious as acid or alkali leaching, can be removed by mechanical method or alkali washing. Atmospheric corrosion is mostly non-limiting, often prone to appear on certain surfaces, such as water vapor easily condensed in the lower temperature area or upper surface. When the atmospheric corrosion is serious, the cross section of the pitting is inverted mushroom-like, at this time alkali washing not only can not eliminate the pitting, but will make it expand.
If the corrosion is determined to be atmospheric corrosion, check the plant's storage conditions. Aluminum should not be stored at the lowest temperature to prevent condensation of water vapor. The storage place should be dry, the temperature is as uniform as possible. 1.3  paper corrosion (water spots)
paper or cardboard is used to place the aluminum or packaging aluminum, so as to avoid abrasions. At this point, if the paper moisture, the surface of aluminum corrosion spots. When the corrugated board is used, there are a few corrosive spots in the position of contacting the corrugated board regularly. Although defects can sometimes be seen directly on aluminum, alkali washing and anodic oxidation are often more pronounced. The spots are generally darker and difficult to remove by mechanical means or alkali washing.
Paper (board) corrosion is caused by acid, mainly the
So2-4 and CL
-, they live in the paper. Therefore, the use of non-chloride and sulfate paper (plate) and avoid water seepage is an effective way to solve the paper (board) corrosion. 1.4  Cleaning Water corrosion (also known as snow-like corrosion) (4)
When the workpiece in the alkali washing, chemical polishing, or sulfuric acid after the removal of ash, water will sometimes appear in the star or radiation-like spots, corrosion depth is very shallow, clean water pollution is serious or overflow washing the flow rate is low prone to this corrosion, because its appearance is similar to snowflake , it is also called snow-like corrosion. The reason is that the
impurity zinc in aluminum and cleaning water so2-4
and cl-reaction. If the trough insulation is not good, then the electric coupling effect will aggravate this flaw. Foreign data introduction, aluminum alloy in zn>0.015%, cleaning water cl->15ppm and ph<3.5 prone to this corrosion. Use nitric acid to remove ash or clean

Water can be removed by adding 0.1%HNO3 (5)
. 1.5  chloride corrosion
The presence of a small amount of chlorides in the sulfuric acid anodic oxidation bath is also prone to pitting, and its appearance is characterized by deep star-shaped black spots, which are more
dense at the corners of the workpiece or other high current density areas. There is no anodic oxide film at the pitting position, and the film thickness of the remaining "normal" areas is lower than the expected film thickness. The high salt content in tap water is the main source of cl-pollution in the trough solution. Foreign data suggest that the cl->200ppm in the trough solution needs to be
handled (5)
. However, it is quite difficult to remove cl-in the contaminated bath fluid. Based on cost considerations, it is more realistic to emit a portion of the tank and add the water with low salinity. 1.6  galvanic corrosion
in the trough (anodic oxidation or electrolytic coloring), due to the workpiece and trough (steel groove) of the electric coupling, or in the non-power of the trough (washing or sealing), due to the role of stray current, will be due to galvanic corrosion caused or aggravated pitting, there are often white sediments around the pitting. The method of solving galvanic corrosion is comparatively simple, and the conductive beam and groove should be insulated with plastic pad, and the steel groove should be grounded, and the conductive rod and fixture should be made with the same material as the workpiece or with small potential difference.
2  appearance unevenness
appearance is actually refers to the stripe and pitting two surface defects other than the surface defects of other types. It not only includes burning loss, white spots, flow marks and other local defects, but also includes surface hair dark (loss of light), powder cream (white) and other large areas of appearance of undesirable defects. 2.1  oxidation burn loss (burnt) ' burn-in ' is caused by an excessive current density (which may be localized) during anodic oxidation, which causes the appearance to scorch. This is the result of partial or widespread catastrophic dissolution of the anodic oxide film, which may occur partially or completely dissolved under the membrane. This is due to the anodic oxidation process can not effectively dissipate heat, causing serious overheating state. The initial burn loss is a local form. On the anodic oxidation film, there is not a small scar, the subsequent dissolution of the stains and burning loss gradually expanded. The cause of overheating may be the electrical contact area is not enough, yin-yang pole spacing is too

Near, the agitation is insufficient or the stirring unevenness causes the temperature not to blend the sulfuric acid electrolyte the aluminum content is too high. 2.2  powder
After anodic oxidation of the surface of the white powder cream called "powder." The powder frost appearing on the surface of the aluminum profile after sealing may be due to "chalking", or it may occur during the sealing process (both the hot sealing hole and the cold seal hole), which should be differentiated. "Powder" surface can be wiped with a damp cloth, but in outdoor use or indoor storage and re-appearance. The
powder is basically caused by the corrosion of the electrolyte by the local overheating, often due to the mixing is too poor and the electrolyte temperature is too high. Therefore, we should analyze the causes of powder and take countermeasures.
If the powder range of the membrane is quite common, it can be considered that the temperature of the electrolyzer is too high, should be checked and adjusted. If the powder occurs in individual profiles or individual positions, it may be too tight nesting or improper electrical contact, it may also be due to the mixing of the cell liquid is not good cause. 2.3  Dark Spots (also known as hot spots or soft spots)
Black spots are anodized after the dark (gray or black) marks, and sometimes in the direction of the extrusion of regular spacing appear, sometimes irregular, generally in the profile of a certain surface appears, in fact, the dark spots in the alkali wash can be seen, However, when the nitric acid is removed, it can be completely disappeared, and the anode is oxidized with gray or black spots, and may appear in the form of white spots or no light stains when the condition is not serious.
Most opinions suggest that the spot is the position where the Mg2si precipitates when the extruded profile is cooled, and the mg2si phase and the matrix are anodes. Therefore, in alkali washing priority corrosion, anodic oxide film structure than normal membrane confusion. The effective measure to eliminate dark spots is to minimize the contact time between the hot extrusion profile and the graphite support block of the discharging port, so that the profile is cooled to about 250 ℃. One of the simplest ways is to increase the cooling air volume forced cooling (4). 2.4  Alkali wash Mark
Alkali washing on the profile to see the flow of caustic soda drying after the corrosion mark, after anodic oxidation easier to see. The reason is that the lye concentration is too high, the lye temperature is too high, and the

The

Alkali bath is transferred to the wash trough and is too slow, when the alkali lotion is concentrated or dried locally on the surface of the
profile to leave the etching mark. 2.5  flashing pattern
After alkali washing, the surface of the aluminum profile flashing grains, showing rough uneven erosion, the larger the size of the extruded material more obvious. This is the selective grain corrosion caused by zinc contamination in alkali lotion. The zinc content in the ingot should be reduced to less than 0.05%, and the concentration of zinc in alkali lotion drops below 3ppm. If the zinc concentration is high, the alkali lotion is added na2s, so that the formation of ZnS precipitated (3). The added amount of na2s is generally twice times the calculated value of ZnS, and it can also reduce the extrusion temperature and prevent coarse grains. 2.6  appearance Dark (also known as hair ash, loss of light)
after anodic oxidation, the surface is out of light and gray, and the tone chaos after electrolytic coloring is a more common defect. There are several reasons for this:
(1) metallurgical factors. LD31 Alloy Mg2si, the ratio of mg/si, impurities (mainly fe,mn, etc.) content and distribution will affect the transparency of anodic oxide film, extrusion process and heat treatment conditions are also affected. High temperature homogenization and aluminum rod heating and other heat treatment, if directly burning oil or natural gas, it produces a large number of hydrogen, water vapor and sulfur dioxide and other harmful gases. Electrolytic coloring forms an matte white point that is caused by hydrogen entering the material. The direct heating should be changed to indirect heating or electric heating at this time.
(2) alkali erosion. Alkali washing liquid alkali concentration is too high or alkali washing temperature is too high, alkali washing time is too long may cause aluminum corrosion, make aluminum surface become rough, thereby losing luster, and affect dimensional accuracy. After coloring, the appearance is cloudy and the hue is not bright. In addition to determining the correct corrosion conditions in the process, additives can also be added to add corrosion inhibitors or the addition of ingredients have a good effect.
(3) anodic oxidation. Although the anodic oxide film of aluminum is transparent, the film thickness will affect the brightness. The higher the film thickness, the lower the brightness. Generally speaking, the real current density is 1a/dm2, and the surface luminance loss is minimal. Control applied voltage
Anodic oxidation often does not control the current density, not only the thickness of the film is difficult to control, but also affect the brightness. The high temperature of sulfuric acid electrolyzer is favorable for the formation of bright film, just the hard

of film.

Degree of decline. In the case of a normal anodizing process, the external
The observation of grey may be related to the pollution of the sulfuric acid cell solution. Heavy metal ions are harmful, such as iron not only reduce the film hardness, but also make the film dark. Impurities such as copper, nickel and manganese have the same effect as iron. Foreign to the above allowable value of heavy metal impurities is 50ppm, the industrial use of sulfuric acid is unavoidable iron pollution, the foreign regulation of sulfuric acid in the allowable value of iron is 20ppm. The source of impurities may also come from the aluminum alloy itself, such as copper-containing aluminum alloy dissolved contaminated trough liquid (4).
(4) Electrolytic coloring. The opacity of the anodic oxide film affects the shading effect. But shades of gray, poor color or not true black may also be due to the electrolytic coloring solution into impurities caused by pollution. The pollution of aluminum ions is common and has obvious unfavorable effects. Electrolytic coloring when a large amount of spilled gas should be tried to adjust the elimination. Japanese data point out that in electrolytic
When the coloring voltage is lower than the anodic oxidation voltage, if the problem is detected,
It is recommended that the electrolytic coloring voltage be increased to (4) above the anode oxidation voltage.

The example lists the occurrence (cause) and discovery operations of 12 appearance defects.
The causes and findings of appearance defects are often not in the same process, and should be noted in the analysis of the cause of defects.

Common defects in anodic oxidation of aluminum alloys