Anodized Aluminum is a type of electrolytic passivation treatment used to increase the thickness of the oxide layer on the surface of metal parts. Generally, aluminum alloys profiles are easily oxidized. Although the oxide layer has a certain passivation effect, the oxide layer will peel off as a result of long-term exposure. Loss of protection, so anodic oxidation is to take advantage of its easy oxidation characteristics to control the formation of oxide layers by electrochemical methods to prevent further oxidation of aluminum and increase the mechanical properties of its surface. Another purpose is to use different chemical reactions. It produces various colors to enhance beauty, and is widely used in aircraft skins, military weapons, photocopier paper rollers, aluminum curtains for buildings, aluminum doors and windows, etc. Anodized aluminum alloy can improve corrosion resistance, increase oxidation color, and improve adhesion. But it can’t increase the strength of aluminum. In addition, the anodic oxide layer is not conductive.

1. History

Origin of the name

The origin of the name of anodizing is that metal parts are placed on the anode in the electronic circuit. Anodizing makes the metal parts less prone to corrosion and wear, and makes the primer more completely attached to the parts. Anodizing provides a variety of surface modification effects. Such as plating on a thicker and porous surface to make the dye easier to absorb, or a thinner transparent layer to increase light reflection

The first large-scale industrial application of the anodic oxidation process was in 1923. The purpose was to prevent the seaplane made of Duralumin from being oxidized and corroded. The early process using chromic acid as the electrolyte was called the Bengough-Stuart process, and this process is still in use today.

In 1927, the chromic acid electrolyte anodic oxidation process was modified into sulfuric acid electrolyte by Gower and O'Brien and registered as a patent. So far, sulfuric acid electrolyte is the most common anodic oxidation method [3].

The oxalic acid anodizing process was registered as a patent in Japan in 1923, and the process was widely used in Germany afterwards, especially in the German construction industry. Anodized aluminum extrusions were once a very popular building material in the 1960s and 1970s, but they were quickly replaced by cheaper plastics and powder coating processes

The latest development in anodizing is the phosphoric acid-based process. Up to now (2020), this process has only been used for pretreatment of binders or organic coatings. Various new anodizing processes are being continuously developed, so the future trend is to classify anodizing processes based on military and industrial standard coating characteristics, rather than chemical reactions of the process

2.Process flow

Chemical degreasing (Na3 PO4 60g/L, Na2CO3 40g/L, 40℃, 3min)—>washing 1>alkali etching (NaOH, 40g/L, 3min)—>washing 1> Idemitsu (HNO3 40g/L ,15s) -Water washing-deionized water washing-anodic oxidation (180g/L H2SO4, current density 1.4A/dm2, oxidation time 30min, temperature 18~22℃)-water washing-sealing (10min) →water washing →blowing dry.

After anodizing, the following four processes are used for sealing:

(1) Boiling water sealing, in boiling pure water (pH = 6.5 ~ 7), sealing for 10 minutes;

(2) Wrong salt blocking, 30g/L potassium fluorocolic acid (pH =4.5~4.6) at room temperature, blocking for 10min;

(3) Blocking with wrong salt, 30/L potassium fluorocolic acid (pH=5.0~5.1) at a medium temperature of 60℃, blocking for 10min;

(4) HB sealing, 6g/L HB (pH = 5.5 ~ 6) at 60 ~ 65 ℃, sealing for 10 minutes

3.Effect

Anodizing is used to avoid sharp corners or burrs after screw threading. It is also used as the dielectric of the electrolyte container. The anode layer is most commonly used to protect aluminum alloys. There are also others such as titanium, zinc, magnesium, niobium, zirconium, hafnium, and tantalum. . Iron and carbon steel will flake off in a neutral or alkaline electrolyte solution. The flakes are iron hydroxide, or rust, which is composed of the surface of the anode cavity and the cathode that lack oxygen, and the cavities gather like sulfuric acid. Anions such as salt and chloride accelerate the rate of rusting of the underlying metal. The carbon flakes or carbon blocks inside the iron block, such as high-carbon steel or pig iron, will interact with the surface coating or electroplating layer. Iron-containing metals are usually placed in a nitric acid solution for anodizing, or fuming nitric acid is used to form a layer of hard black iron oxide.

4.Processing method

Surface pretreatment

In the pretreatment of the aluminum alloy surface, the main purpose is to remove the oil and impurities on the surface, so as to ensure the cleanness of the surface, and also to make the state of the sample meet the requirements. First, according to the 1:1 requirement, use absolute ethanol and deionized water to achieve the corresponding configuration of the cleaning solution; second, place the cut sample directly in the beaker with the cleaning solution and place it Dedicated to the inside of the ultrasonic cleaner, clean it for 5 minutes; after the cleaning is completed, wipe it clean with filter paper, and then choose to use 240#, 400#, 600#, 800#, 1200#, 2000# sandpaper for polishing ; Third, clean the sample in distilled water, then place it in a beaker (with acetone solution), clean it in a washing machine for 10 minutes, then take out the sample and dry it directly.

Preparation of anodic oxide film

Add the sample directly to the anodizing device (pre-treatment), select 100g/L sulfuric acid solution, set the oxidation voltage to 8, 10, 12, 14, 16V, and finally obtain the corresponding sample to Prepare for subsequent analysis.

Sealing treatment

Add an appropriate amount of deionized water to the beaker, and then put it into the water bath to directly heat and boil, and finally put the sample in the deionized water for sealing treatment, and then require a standing treatment for 20 minutes. Take out the sample, rinse with deionized water, and wait for it to air dry.

Performance Testing

In the performance test, two aspects are selected this time: first, metallographic observation. If you choose to use a metallurgical microscope, you can analyze and observe the anodized film and the area that is not coated. Second, oxide film thickness and spot experiments. It is mainly to measure the thickness of the oxide film and analyze the spot experiment.

5.Experiment

The experimental material and its pretreatment The experimental material is a 5052 aluminum alloy sample, and its chemical composition is shown in the chart below.

Before the experiment, polish with 1200 grit sandpaper to remove the oxide film that is naturally formed on the surface of the sample when exposed to the air, and then soak it in an alkaline solution for about 10 minutes. After degreasing, pickling is carried out, and the oil, dust and oxide film on the surface of the sample are completely removed by the etching effect of acid. The surface condition of the sample after pickling meets the requirements, and oxalic acid anodization and chromic acid anodization can be carried out after cleaning and drying. The following chart shows the composition and process conditions of degreasing and pickling solutions.

Oxalic acid anodizing and chromic acid anodizing

Using a DC anodizing power supply, the treated 5052 aluminum alloy sample is placed in the electrolyte as the anode, and an oxide film is formed on the surface of the sample based on the principle of electrolysis after being energized. The electrolyte composition and process conditions of oxalic acid anodizing are: oxalic acid 50 g/L, voltage 45V, current density 1.4A/dm2, electrolyte temperature 25 ℃, time 55 min.

The electrolyte composition and process conditions of chromic acid anodization are: chromic acid 40g/L, voltage 40V, current density 0.75A/dm2, electrolyte temperature 35°C, time 55min.

Morphology characterization and performance test of oxalic acid anodic oxide film and chromic acid anodic oxide film

The morphology of the oxalic acid anodic oxide film and chromic acid anodic oxide film was characterized by Hitachi S-4800 scanning electron microscope. The surface roughness of the oxalic acid anodic oxide film and the chromic acid anodic oxide film were measured by SJ-210 roughness meter. Four positions were taken and the measurement results were recorded respectively. Use the Princeton PARSTAT2273 electrochemical workstation to test the polarization curves and impedance spectra of the oxalic acid anodic oxide film and the chromic acid anodic oxide film. The electrode system is a three-electrode system: the platinum electrode is the auxiliary electrode, and the saturated calomel electrode is the reference electrode. The oxalic acid anodic oxide film sample and the chromic acid anodic oxide film sample were used as working electrodes, and both were tested in sodium chloride solution (3.5wt.%). The scanning rate of the polarization curve test is 0.5mV/s, and the impedance spectrum test scans from 105 Hz in the high frequency area to 10-2 Hz in the low frequency area.

Thickness of oxalic acid anodic oxide film and chromic acid anodic oxide film

The thickness of the anodic oxide film refers to the distance from the outer surface of the anodic oxide film to the inner surface (ie the interface between the anodic oxide film and the substrate). The thickness has a great influence on the performance of the anodic oxide film (such as corrosion resistance, bending resistance, etc.) Influence.

The morphology of oxalic acid anodic oxide film and chromic acid anodic oxide film. The anodic oxide film mainly plays a decorative and protective effect on aluminum alloy, so the morphological quality of the anodic oxide film is particularly important. Generally speaking, the topography quality of anodized film mainly includes color, surface roughness and surface defects. The appearance of the oxalic acid anodic oxide film and the chromic acid anodic oxide film was observed with naked eyes. The former was light gray and the latter was silver-white. The two anodic oxide films had no surface defects on the macro scale.

Corrosion resistance of oxalic acid anodic oxide film and chromic acid anodic oxide film

The following figure shows the polarization curves of 5052 aluminum alloy sample, oxalic acid anodic oxide film and chromic acid anodic oxide film. It can be seen from the figure below that the corrosion potentials of the oxalic acid anodic oxide film and the chromic acid anodic oxide film are -412.6 mV and -645.7 mV, respectively, which are higher than the corrosion potential of the 5052 aluminum alloy sample (-750.4 mV). The Tafel curve extrapolation method was used to fit the polarization curve. In addition, the corrosion current densities of oxalic acid anodic oxide film and chromic acid anodic oxide film were 1.31×10-5 A/cm2, 1.70×10-5A/cm2 Compared with the 5052 aluminum alloy sample, the corrosion current density is significantly lower. The corrosion current density can theoretically characterize the corrosion rate of the tested material, and there is a conversion relationship between the two. Generally speaking, the smaller the corrosion current density, the slower the corrosion of the tested material. Therefore, the order of corrosion resistance is: oxalic acid anodic oxide film>chromic acid anodic oxide film>5052 aluminum alloy sample.