Aluminum alloys are widely used in the transportation industry due to their excellent mechanical properties, light weight, and good machinability. However, in complex service environments, aluminum alloys are susceptible to corrosion and wear, which can impact both performance and operational safety.
Factors Leading to Aluminum Corrosion
The main factors influencing the corrosion behavior of aluminum alloys can generally be divided into three categories. They are environmental factors, load-related factors and material-related factors.
Environmental Factors
Metal corrosion is the result of interactions between metallic materials and their surrounding environment. Temperature, humidity and industrial pollutants such as sulfur dioxide are among the major causes of aluminum alloy corrosion.
Transportation equipment operating in high-humidity tropical climates is more likely to experience corrosion problems. In industrial regions, exposure to pollutants can accelerate the corrosion of aluminum alloy components. In marine environments, chlorides can trigger localized corrosion such as pitting corrosion and stress corrosion cracking.
Research data shows that after five years of exposure in an industrial-marine environment, the corrosion layer thickness on aluminum alloys can reach 136.81 μm, while the corrosion penetration depth may reach approximately 125.46 μm. During this process, yield strength and tensile strength may decrease by 24.5% and 21.1%, respectively.
Load-Related Factors
Load-related corrosion mainly appears in the form of stress corrosion and corrosion fatigue. As transportation systems operate at higher speeds, critical components are subjected to greater stress, increasing the risk of stress corrosion.
Experimental results show that when a constant tensile stress of 304.5 MPa is applied, the corrosion potential of aluminum alloy decreases from -792 mV to -824 mV, while the corrosion current density increases from 2.1867 μA/cm² to 17.516 μA/cm².
In addition, continuous vibration and cyclic fatigue can lead to the formation of microcracks. These cracks often become starting points for corrosion, reducing mechanical performance and leading to stress corrosion cracking.
Material Factors
Material-related factors mainly include galvanic corrosion and the influence of alloying elements. In transportation equipment, contact between dissimilar metals can lead to galvanic corrosion.
For example, when aluminum machined parts fit with copper alloys, magnesium alloys or stainless steel fasteners, the aluminum alloy that has a lower electrode potential will act as the anode and undergoes oxidation corrosion. Experiments have confirmed that after Al/Cu welded joints are immersed in a 3.5% NaCl solution for 24 hours, pitting corrosion appears on the aluminum side while the copper remains largely unaffected.
In addition, alloying elements such as Cu, Mg, Mn and Si can form precipitated phases with different electrode potentials, leading to localized corrosion. In copper-rich aluminum alloys, copper-containing intermetallic compounds act as cathodes and accelerate corrosion. In magnesium-rich aluminum alloys, magnesium intermetallic compounds act as anodes and corrode preferentially, resulting in pitting and other localized corrosion phenomena.
Main Finishing Technologies for Aluminum Alloys
Metal fabrication manufacturers have introduced many surface treatment methods to enhance the corrosion resistance of aluminum alloys. And they can be divided into six categories. They are chemical conversion coatings, anodizing, organic coatings, electroplating, laser cladding and sol-gel methods.
Each technology has its own advantages and limitations in terms of corrosion resistance, wear resistance and durability. Therefore, the appropriate process should be selected according to the specific application environment and performance requirements.
Chemical Conversion Coatings
Chemical conversion coatings form a tightly bonded protective layer through reactions between metal surface atoms and anions in the treatment medium. Common types include chromate coatings, oxalate coatings, phosphate coatings and oxide coatings.
Traditional hexavalent chromate conversion coatings (CCC) provide excellent corrosion resistance. However, due to the environmental and health hazards associated with hexavalent chromium, their use has been heavily restricted.
To address this issue, trivalent chromium conversion coatings (TCP) and chromium-free coatings such as zirconate, titanate and molybdate systems have been developed. These newer coatings maintain good corrosion resistance while reducing environmental and health risks, making them a mainstream choice in modern aluminum surface treatment.
Anodizing
Anodizing is a process that forms a dense oxide film on the surface of aluminum alloys through anodic current in an electrolyte solution. This process can significantly improve corrosion resistance.
Common electrolytes include sulfuric acid, chromic acid, oxalic acid and phosphoric acid. Among them, sulfuric acid anodizing is the most widely used because of its mature process and relatively low cost.
The oxide layer produced by sulfuric acid anodizing features a dual-layer structure with good corrosion resistance, wear resistance and decorative properties. However, anodized films contain pores, making them vulnerable to corrosive media. To solve this issue, sealing treatment is usually required.
Current sealing methods mainly include hot water sealing and metal salt sealing, with nickel salts and dichromates being widely used because of their effectiveness.
Manufacturers have also developed various modification methods to further improve the corrosion resistance of anodized films. By adding rare earth elements, metal particles or oxide particles into the electrolyte, the structure and properties of the oxide layer can be enhanced.
Organic Coatings
Organic coatings are considered one of the most economical and effective methods for protecting aluminum alloys against corrosion. Their development has evolved from single-layer coatings to complete coating systems that include primers and topcoats.
At present, acrylic resins and epoxy resins are among the most widely used organic coating materials. However, studies have shown that the adhesion strength of organic coatings can be affected by ultraviolet radiation, ozone and moisture exposure.
To improve performance, modified fillers such as zirconium-based or nickel-based fillers can be added to significantly enhance corrosion resistance. In addition, organosilanes can improve bonding strength between the coating and the metal surface, enhancing both adhesion and corrosion protection.
It should be noted that traditional organic coatings still present environmental concerns. Many contain volatile organic compounds (VOCs) and chromate fillers, which may lead to risks to both the environment and human health.
Electroplating
Electroplating is a surface treatment process that deposits metal or alloy films onto aluminum alloys through electrochemical methods.
Chrome plating can be divided into decorative chrome plating and hard chrome plating. Hard chrome plating can improve surface hardness, wear resistance and corrosion resistance.
Copper plating is often used as a pretreatment process because of its excellent adhesion properties. Nickel plating improves both decorative appearance and corrosion resistance, helping extend service life.
In zinc plating, potassium chloride zinc plating has become one of widely used processes because of its good electrical conductivity, lower bath voltage and easier wastewater treatment.
However, electroplating wastewater often contains heavy metals and other hazardous substances that may affect the environment and human health if not properly treated.
Laser Cladding
Laser cladding is a surface modification technology based on high-energy-density laser beams. It improves surface performance by melting and depositing wire or powder materials onto aluminum alloy surfaces.
The technology mainly includes three process types: coaxial powder/wire feeding, pre-placed powder and off-axis powder/wire feeding. Among them, coaxial powder or wire feeding offers several advantages, including higher process stability, better precision, cleaner working conditions and denser cladding layers with fewer defects.
Laser cladding can effectively improve the corrosion resistance and wear resistance of aluminum alloys. However, defects such as pores and cracks may still occur during practical applications.
Studies have shown that adding rare earth elements or alloying elements such as Mn, Cu and Ti can significantly improve the overall performance of the cladding layer.
