5 Main Types of Aluminum Corrosion

Corrosion of metal can compromise its functional strength, leading to significant structural degradation, including cracking, partial component fracture, and, in extreme cases, total material failure. Aluminum and its alloys are widely used across diverse industries due to their high strength-to-weight ratio and inherent passivity. In this article, we will discuss the 5 main types of aluminum corrosion and how to prevent them from happening.

What is Aluminum Corrosion?

Aluminum corrosion is the gradual degradation of aluminum molecules into their oxides, which diminishes the material’s physical and chemical properties. Inherently, aluminum is a reactive metal, yet it is also a passive metal. This means that while nascent aluminum will react with oxygen and water in the environment, the resulting compounds form a protective layer on the surface that shields the underlying material from further degradation. This non-reactive oxide layer adheres well to the surface and is not easily shed, like stainless steel. Unlike deliberate processes such as laser etching and aluminum anodizing, corrosion is a slow process that can take months or years.

Types of Aluminum Corrosion

The degree and form of aluminum corrosion depend not only on the corrosive medium but are also closely related to the composition and temper of the aluminum alloy. In typical service environments, the primary forms of corrosion for aluminum and its alloys include pitting corrosion, intergranular corrosion, exfoliation corrosion, stress corrosion cracking (SCC), and galvanic corrosion.

Pitting Corrosion

Pitting corrosion is the most common form of degradation observed in aluminum and aluminum alloys. Generally, pitting of aluminum alloys in the atmosphere is not severe, but it is significantly more aggressive in water, potentially leading to perforation in a relatively short period. Conditions conducive to pitting include the presence of ions that inhibit general corrosion (such as SO₄^2-, SiO₃^2-, and PO₄^2- alongside ions that locally destroy the passive film (such as Cl^–), as well as the presence of accelerants (oxidizers) that promote the cathodic reaction.

To prevent pitting in aluminum alloys, environmental control measures often involve removing oxidizing ions, such as chloride ions, from the medium. The presence of copper ions in water is a common cause of aluminum pitting, so minimizing copper content in the water is essential.

High-purity aluminum is resistant to pitting. However, the content and distribution of impurities in commercially pure aluminum greatly affect its pitting resistance. Copper-containing aluminum alloys exhibit the lowest resistance to pitting. Aluminum-manganese and aluminum-magnesium alloys can achieve better pitting resistance, provided their processing quality is maintained.

Intergranular Corrosion

Intergranular corrosion (IGC) refers to corrosion damage that initiates and develops along or immediately adjacent to the grain boundaries of a metal in a specific corrosive environment. The physicochemical state of the grain boundaries differs from the grain interior. In specific corrosive media, this difference leads to localized, selective corrosion damage due to micro-galvanic cell action. In practical applications, pure aluminum does not experience IGC. Alloys in the Al-Cu, Al-Cu-Mg, and Al-Zn-Mg-Cu systems exhibit significant susceptibility to IGC. Inappropriate heat treatment can lead to a severe propensity for IGC. Specifically, when the copper-rich CuAl₂ phase precipitates continuously along the grain boundaries, copper-depleted zones are simultaneously formed adjacent to the boundaries. A corrosion cell is established where the CuAl₂ and the copper-depleted zone form the cathode and anode, respectively. The copper-depleted zone corrodes, resulting in IGC.

Furthermore, grain boundary precipitation can occur in other alloy systems. For instance, coarse precipitates of the magnesium-rich phase may appear at the grain boundaries of high-magnesium aluminum alloys. This phase acts as the anode, is selectively damaged, and causes IGC. These conditions are sensitive to the alloy’s processing and heat treatment. Correct processing can render sensitive alloys less susceptible, while unsuitable processing can cause severe IGC even in alloys that were initially not highly sensitive.

Appropriate heat treatment to eliminate the continuous precipitation of deleterious phases along grain boundaries can effectively mitigate the tendency for IGC.

Exfoliation Corrosion

Exfoliation corrosion is a specific form of corrosion that affects aluminum alloys. It is characterized by lateral propagation along grain boundaries parallel to the surface of the alloy profile. This damage manifests as powdering, peeling, blistering, or even delamination. Exfoliation significantly reduces the strength and ductility of the aluminum alloy, thereby shortening the structure’s service life. Exfoliation corrosion often occurs when aluminum alloy material has not been annealed or has been insufficiently annealed. Aluminum alloys that undergo stabilization treatment often exhibit a higher susceptibility to exfoliation corrosion. When environmental conditions, such as temperature, are met, exfoliation corrosion initiates, localized corrosion accelerates, and the alloy suffers severe damage with a characteristic layer-by-layer peeling morphology.

Key measures to mitigate or eliminate exfoliation corrosion in aluminum alloys include:

• Adding grain-refining transition elements such as Cr, Mn to inhibit or slow the precipitation of second phases along grain boundaries and promote their uniform distribution both within the grains and at the boundaries. This disrupts the anodic pathway for corrosion development.
• Incorporating elements that reduce the potential difference between phases, such as adding a controlled amount of Zn to Al-Mg alloys.
• Using appropriate heat treatment procedures.
• Using cathodic protection via sacrificial anodes.

Stress Corrosion Cracking

Stress corrosion cracking (SCC) refers to the delayed cracking or fracture of stressed aluminum alloy materials in specific media. This phenomenon is a synergistic result of the combined action of a corrosive medium and mechanical stress.

SCC primarily occurs in hard aluminum and ultra-hard aluminum alloys. Pure aluminum and low-strength aluminum alloys generally do not exhibit a propensity for SCC. SCC in aluminum alloys is always intergranular. Aluminum alloys that are susceptible to IGC are also sensitive to SCC. SCC frequently occurs when aluminum alloys are exposed to the atmosphere, particularly near-coastal atmospheres and seawater. Sensitivity to SCC increases with higher temperature and humidity, higher chloride ion concentration, and lower pH values.

Measures to prevent or eliminate SCC in aluminum alloys include:

• Heat treatment to relieve internal stresses.
• Alloying with trace elements such as Mn, Cr, V, Zr, and Mo to improve resistance to tensile SCC.
• Bead blasting to modify the surface stress state (introducing beneficial compressive residual stresses).

Galvanic Corrosion

Galvanic corrosion affects aluminum when it is physically or electrolytically connected to a more noble (less reactive) metal. A noble metal is any metal that has a lower reactivity compared to aluminum. A metal’s reactivity is determined by its position in the electrochemical series. The farther away another metal is from aluminum in the galvanic series, the more severe the corrosion will be. The corrosion intensity is highest at the interface where the two metals meet and weakens as the distance from this interface increases. For example, if aluminum and brass are in contact or even in proximity and placed in seawater, a galvanic cell is formed. The aluminum component will then corrode because it acts as the anode. This can be a significant issue in vessels where brass fittings may be near aluminum fittings, and both are immersed in seawater. Electrons flow from the aluminum (anode) to the brass (cathode) through the seawater (electrolyte).