Causes of aluminum alloy corrosion and its protective measures

Current status of aluminum alloy corrosion

As a metal material with superior performance, aluminum alloy has been widely used in many fields such as shipbuilding and high-speed trains. However, aluminum alloy faces serious corrosion problems in different environments.

In a dry atmospheric environment, the passivation film on the surface of aluminum alloy is stable and not easy to be destroyed. However, local pitting will occur if it is exposed to the outdoor atmospheric environment for a long time. For example, after dust ions are deposited on the surface, an oxygen-deficient zone is formed on the metal surface in the water film under the dust ions, resulting in the destruction of the passivation film and the decrease of self-passivation ability.

Aluminum alloy corrosion

In the industrial atmosphere, the protective film is easily damaged and the corrosion resistance decreases. Especially in areas polluted by sulfur oxide acid rain, the corrosion resistance decreases significantly, and the front of the aluminum material is generally black, black with white spots or gray with black spots.

In the marine atmosphere, CL- has a strong destructive effect on the passivation film. The passive state of aluminum alloy in seawater is unstable, and local corrosion is its main corrosion form. Common local corrosion includes pitting and crevice corrosion. Pure aluminum does not produce intergranular corrosion, while aluminum alloy has a greater sensitivity to intergranular corrosion. Stress corrosion mainly occurs in heat-treated high-strength aluminum alloys, and all of them are intergranular cracking type. When aluminum alloys are in contact with most metals in seawater, they are anodic, which will accelerate the corrosion of aluminum. In the full immersion area or tidal range area, the surface marine biofouling is more serious than other metals, which will aggravate the local corrosion of aluminum alloys.

Under different corrosion environments, the average pitting degree of aluminum alloys in 20 years is serious. In rural environments, it is 10~55μm; in urban environments, it is 100~190μm; in marine environments, it is 85~260μm. When aluminum is in contact with metals such as steel, copper and stainless steel, there is a risk of galvanic corrosion.

The corrosion problem of aluminum alloys not only affects its aesthetics, but also reduces its strength and service life, and even threatens its safety in use. For example, in ship construction, the corrosion of aluminum alloy structures may cause piercing of the hull, affecting the navigation safety of the ship; in high-speed trains, the corrosion of aluminum alloys may affect the running stability and safety of the train. Therefore, it is crucial to solve the corrosion problem of aluminum alloys.

The “natural enemy” of aluminum alloy corrosion

(I) Fear of chemical substances

Aluminum alloys are very sensitive to chemical substances such as alkali, acid, and chloride, and are prone to corrosion reactions. When aluminum alloys encounter strong alkaline substances, such as sodium hydroxide, the protective film on its surface will be destroyed, and aluminum will dissolve in it, thereby being corroded by oxygen. Dilute sulfuric acid will also corrode the dense protective film on the surface of aluminum alloys. The chemical formula is Al₂O₃+3H₂SO₄═Al₂(SO₄)₃+3H₂O. In addition, one of the corrosions that aluminum alloys fear most is chloride. Although aluminum alloys are the preferred material in the manufacture of marine equipment, bridges, automobiles, and ships, chloride salt water has a strong corrosive effect on aluminum alloys. When the surface of aluminum alloys is contaminated by chloride salt water, it will lose its toughness due to the attack of chlorides and is easily damaged by repeated mechanical stress.

Causes of aluminum alloy corrosion

(II) Concerns about inferior profiles

Impurity-containing recycled aluminum profiles produced by informal manufacturers are a major hidden danger of aluminum alloy corrosion. If the aluminum profiles used for aluminum alloy doors and windows are not produced by regular manufacturers and do not meet national standards, but recycled aluminum containing impurities is used to fish in troubled waters, then such aluminum doors and windows are prone to rust and corrosion. This inferior profile has defects in manufacturing process and material quality, and its surface oxide layer is easily damaged, which cannot provide effective protection for aluminum alloy, thereby increasing the risk of rust and corrosion of aluminum alloy.

(III) Risks of special environments

In special environments such as oceans and industries, aluminum alloys face severe corrosion challenges. In marine environments, aluminum alloys are susceptible to oxidation, sulfide, and chloride corrosion. The corrosion of aluminum alloys in the ocean is mainly due to oxidation reactions and the action of chloride ions. Chloride ions in seawater will penetrate into the surface of the alloy and form an oxide layer with the aluminum surface. However, under long-term exposure, chloride ions will destroy the oxide layer, resulting in aggravated corrosion of aluminum alloys. At the same time, sulfide is also a major “natural enemy” of aluminum alloys in the marine environment. Sulfide is a compound with extremely strong electron affinity, which can easily corrode the surface of aluminum alloys. In the field of aviation and aerospace, when aluminum alloy parts used in rockets are operated in remote, oxygen-poor environments, sulfides will form a stable coating on the surface of the material, seriously affecting the performance of the material. In industrial environments, especially in areas polluted by sulfur oxide acid rain, the protective film of aluminum alloys is easily damaged and the corrosion resistance is reduced. The front of aluminum materials is generally black, black with white spots or gray with black spots.

Causes of Aluminum Alloy Corrosion

(I) General corrosion and local corrosion

From the appearance of corrosion, aluminum corrosion can be divided into general corrosion and local corrosion. General corrosion is also called overall corrosion or uniform corrosion, which refers to the uniform corrosion and loss of the surface of the material in contact with the environment. The corrosion of aluminum in alkaline solution is a common uniform corrosion, such as alkali washing. The corrosion result is that the aluminum surface becomes thinner at a similar rate and the weight is reduced. But absolutely uniform corrosion does not exist, and the thickness reduction is different in different places.

Aluminum alloy pitting

Local corrosion refers to the occurrence of corrosion limited to a special range or position of the structure. There are mainly the following types:

1. Pitting: Pitting occurs in a very local range or position of the metal surface, resulting in caves or pits and extending inward, and even causing perforation. Aluminum is often pitted in aqueous solutions containing chlorides. Among the corrosion of aluminum, pitting is the most common, which is caused by the difference between the potential of a certain range of aluminum and the potential of the substrate, or by the presence of impurities with a different potential from the potential of the aluminum substrate.

Aluminum alloy intergranular corrosion

2. Intergranular corrosion: A type of selective corrosion that occurs at the grain boundaries of metals or alloys when the grains or crystals themselves are not significantly corroded, which will cause a sharp drop in the mechanical properties of the material, leading to structural damage or accidents. This type of corrosion can occur in high-purity aluminum in hydrochloric acid and high-temperature water. Al-Mg, Al-Zn-Mg, AI-Mg-Si, and AI-Cu alloys are relatively sensitive to intergranular corrosion. The reason for intergranular corrosion is that the grain boundaries are very active under certain conditions, such as impurities at the grain boundaries, or an increase or decrease in a certain alloying element at the grain boundaries. In other words, there must be a thin layer on the grain boundaries that is electronegative to the rest of the aluminum, and it corrodes first.

3. Galvanic corrosion: When a relatively active metal such as aluminum (anode) touches a less active metal in the same environment or is connected by a conductor, a galvanic couple is formed and current flows, leading to galvanic corrosion. The natural potential of aluminum is negative. When aluminum touches other metals, aluminum is always anode, and corrosion is accelerated. Almost any aluminum and aluminum alloy is difficult to avoid galvanic corrosion. When the potential difference between the two metals in contact is greater, the galvanic corrosion is more obvious. In galvanic corrosion, the area factor is extremely important, and a large cathode and a small anode are the most unfavorable combination.

Aluminum alloy galvanic corrosion

4. Crevice corrosion: When the same or different metals touch each other, or metal and non-metal touch each other, a gap will be formed, and corrosion will be formed at the gap or its vicinity. There is no corrosion outside the gap, which is caused by the lack of oxygen in the gap, because a concentration cell is formed at this time. Crevice corrosion has almost nothing to do with the type of alloy, and even very corrosion-resistant alloys will occur. The acidic environment at the top of the gap is the driving force of corrosion. It is a type of corrosion under deposits (scale). The corrosion under the mortar on the surface of 6063 alloy architectural aluminum profiles is a very common type of crevice corrosion under scale.

5. Stress corrosion cracking: Corrosion cracking caused by the coexistence of tensile stress and special corrosive media. Stress can be external or residual stress inside the metal. The latter may be formed by deformation during processing and manufacturing, or by drastic temperature changes during quenching, or by volume changes caused by changes in internal structure. The stress caused by riveting, bolting, press-fitting, and shrink-fitting is also residual stress. When the tensile stress of the metal surface reaches the yield strength Rpo.2, stress corrosion cracking will occur. Whether it is 7000 series aluminum alloy thick plate or 2000 series, residual stress will be formed during quenching. It should be eliminated by pre-stretching before aging treatment to avoid deformation or even bringing it into the parts during processing of aircraft parts.

Stress corrosion cracking

6. Layered corrosion: This corrosion is also called peeling, flaking, and layered corrosion, which can be simply referred to as peeling. It is a special type of corrosion form of 2000 series, 5000 series, 6000 series, and 7000 series alloys. It is more common in extruded materials. Once it appears, it can be peeled off layer by layer like mica.

Laminar corrosion

7. Filiform corrosion: It is a sub-film corrosion that develops under the film in a worm-like shape. This film can be a paint film or other layers. It generally does not occur under the anodized film. Filiform corrosion is related to alloy composition, pre-coating pretreatment, and environmental factors. Environmental factors include humidity, temperature, chloride, etc.

Filiform corrosion

(II) Analysis of influencing factors

The influencing factors of aluminum alloy corrosion are mainly environment, metallurgy and stress.

• 1. Environmental factors: The environmental factors that affect stress corrosion of aluminum alloy mainly include ion type, ion concentration, solution pH, oxygen and other gases, corrosion inhibitors, ambient temperature, ambient pressure, etc. For example, in different atmospheric environments, the stress corrosion sensitivity of 2A12 and 7A04 aluminum alloys is different, and they are more sensitive in marine environments. The marine environment contains a large amount of salt, and Cl- will penetrate the protective film on the surface of the aluminum alloy and enter the interior, causing corrosion to it. When the mass concentration of HNO3 solution is between 20% and 40%, the corrosion of aluminum alloy intensifies, and the corrosion rate of aluminum alloy reaches the highest point when the concentration is about 35%. In concentrated HNO3 solution, the stress corrosion of aluminum alloy is not obvious, because a dense oxide film is formed on the surface of the aluminum alloy, which prevents further corrosion of HNO3.
• 2. Metallurgical factors: Metallurgical factors mainly include casting method, processing method and heat treatment. Different metallurgical factors change the type of aluminum alloy surface film, and cause different internal organizations and changes in crystal structure of aluminum alloy, thus affecting the electrochemical behavior and mechanical behavior of aluminum alloy, resulting in different stress corrosion sensitivity of aluminum alloy. For example, cathodic polarization increases the stress corrosion sensitivity of aluminum alloy, and the stress corrosion sensitivity of friction stir welding is lower than that of fusion welding. It is generally believed that properly treated 6061-T6 and 3004 aluminum alloys will not have SCC.
• 3. Stress factors: Stress factors mainly include load type, load size, loading direction, loading speed, etc. As far as SCC is concerned, the stress direction must be perpendicular to the grain boundary so that it can be separated. One of the key factors in the generation of stress corrosion is stress. Different stress effects will produce different effects. Alternating stress and environment work together to produce corrosion fatigue, which is usually significantly different from stress corrosion cracking caused by fixed stress. Usually, corrosion fatigue has more serious consequences than stress corrosion. In addition, different loading speeds will also affect the sensitivity of aluminum alloy stress corrosion.

Aluminum alloy protection method

(I) Improving the corrosion resistance of materials

Choosing reasonable composition and heat treatment process is an important method to improve the corrosion resistance of aluminum alloy. For example, some corrosion-resistant elements such as copper, magnesium, zinc, etc. can be added to aluminum alloy to form a corrosion-resistant alloy. These elements can improve the corrosion resistance of aluminum alloy and improve its stability in harsh environments. At the same time, a reasonable heat treatment process can change the internal organization and crystal structure of aluminum alloy, thereby improving its corrosion resistance. For example, through solution treatment and aging treatment, the strengthening phase in aluminum alloy can be evenly distributed, improving its corrosion resistance.

Aluminum alloy anti-corrosion

(II) Surface treatment strategy

• 1. Anodizing: Anodizing is a method of forming a thick oxide film on the surface of aluminum alloy, which can effectively improve the corrosion resistance of aluminum alloy. The anodizing process can adjust the thickness and quality of the oxide film by controlling the process parameters to further enhance its corrosion resistance.
• 2. Painting: Painting is a common surface treatment method that can form a protective film on the surface of aluminum alloy to prevent aluminum alloy from contacting the external environment, thereby improving its corrosion resistance. When choosing a coating, a coating with good corrosion resistance and adhesion should be selected.
• 3. Pure aluminum coated on the surface of hard aluminum: Pure aluminum coated on the surface of hard aluminum can improve the corrosion resistance of hard aluminum. Pure aluminum will form a dense oxide film in the air, which can effectively prevent the aluminum alloy from contacting the external environment, thereby improving its corrosion resistance.

(III) Waterproof and dustproof measures

• 1. Product design: Product design must comply with the IP level to prevent failure to meet the IP detection requirements. In the early stage of design, it is crucial to set the goal that the aluminum shell needs to achieve the ideal protection level.
• 2. Material selection: Choose anodized aluminum. Aluminum alloy will form an oxide film after contacting with air, which can play a role in isolating the air. If other related processes are carried out later, the advantages of the oxide film will be strengthened.
• 3. Component selection: In the component structure of the shell, the waterproof and dustproof effect can be improved through several protective design components. For example, the sealing silicone ring has high-quality sealing performance, tensile resistance, corona resistance, arc resistance, high and low temperature resistance, and good electrical insulation performance.
• 4. Repeated testing: The waterproof shell produced needs to be tested according to relevant standards to achieve socially recognized qualifications. The Yonggu L/M series waterproof aluminum housing has obtained the IP68 certificate certification from Shenzhen Bureau Veritas International Testing Group (currently tested to be completely fine for immersion in 1.5 meters of water for half an hour).

(IV) Cathodic protection technique

Cathode protection is a method of preventing metal corrosion by providing electrons to the protected metal to make it a cathode. In the protection of aluminum alloys, sacrificial anode protection is a commonly used cathodic protection method. Sacrificial anodes usually use metals such as zinc, magnesium, and aluminum. These metals have a more negative potential than aluminum alloys. They are preferentially dissolved as anodes in corrosion cells, release electrons, and flow to the protected aluminum alloy, making it a cathode, thereby preventing aluminum alloy corrosion. For example, in the marine environment, the metal structure of the ship is susceptible to corrosion, and the aluminum alloy sacrificial anode can effectively extend the service life of the ship. Large marine engineering structures such as offshore platforms and submarine pipelines are in seawater and marine atmospheric environments for a long time, and aluminum alloy sacrificial anodes can also provide them with reliable cathodic protection.

(V) Zinc phosphating method

Zinc phosphating is a method of forming a phosphating film on the surface of aluminum alloys, which can improve the corrosion resistance of aluminum alloys. The process of zinc phosphating includes degreasing, rust removal, alkaline etching, acid etching, phosphating, washing and drying. During the phosphating process, the surface of the aluminum alloy reacts with zinc dihydrogen phosphate, nitrate, phosphoric acid and other components in the phosphating solution to form a phosphating film. This phosphating film has good corrosion resistance and adhesion, and can effectively prevent aluminum alloy corrosion. For example, in the surface protection of aluminum alloy chassis, zinc phosphating can be used to improve the corrosion resistance and service life of the chassis.

Summarize

Aluminum alloys are widely used in modern industry, but corrosion problems seriously affect their performance and service life. This article analyzes the natural enemies, causes and protection methods of aluminum alloy corrosion to provide a reference for solving aluminum alloy corrosion problems. Improving material corrosion resistance, surface treatment, taking waterproof and dustproof measures, using cathodic protection and zinc phosphating methods can effectively reduce aluminum alloy corrosion and extend its service life.