There are many reasons why aluminum is the material of choice for custom aluminum profiles, including its lightweight yet strong and easy to machine, and the natural corrosion resistance of aluminum, plus the ability to prevent corrosion through design makes it a very valuable metal material for many different industries.
But, even with these benefits, users’ concerns consider that corrosion can destroy the finish and over time weaken the structure of the metal.
Does Aluminium Corrode?
In most environments, untreated aluminum has very good resistance to corrosion. The natural corrosion resistance of aluminum exists because of the way that aluminum spontaneously forms a thin oxide layer that prevents oxidation. The aluminum oxide coating is highly resistant and renews itself if damaged keeping the metal relatively safe from corrosion. But some factors can cause the coat to become unstable, thus exposing the metal. The most common type of corrosion in aluminum is galvanic corrosion. It happens when aluminum comes in contact with other metals.
If the oxide layer is damaged mechanically, it will begin to repair itself right away. It is because of this extremely beneficial oxide layer that aluminum is the metal of choice for many products. But in environments that are strongly alkaline or acid, it will corrode rather quickly, as if it didn’t have that layer of protection at all. High levels of pH can break down the aluminum oxide coating. In this case, the coat is broken down faster than it can repair itself. To avoid corrosion, untreated aluminum should be used in areas with a pH of between 4.5 and 8.5. If aluminum is exposed to very strong acid or alkaline environments outside the pH range of 4 to 9, violent corrosion will occur in the form of metal pitting.
Common Three Types of Corrosion
There are three major types of corrosion that metal workers and manufacturers have to consider when creating products or materials from aluminum. Those three types include galvanic corrosion, crevice corrosion, and pitting.
Galvanic corrosion occurs when metals break down each other. The least noble metal will become the anode and suffer from galvanic corrosion, while the noblest metal will become the cathode and becomes protected against galvanic corrosion. Aluminum is the least noble metal in most combinations with other metals, posing a greater risk of galvanic corrosion than other typical metals used in structural construction.
For galvanic corrosion to occur, three conditions must all be present:
- Electrochemically dissimilar metals must be present.
- Metals must be in electrical contact.
- Metals must be exposed to an electrolyte.
Galvanic corrosion may also be known as bimetallic corrosion and dissimilar metal corrosion. It does not occur in dry, indoor atmospheres, but is most likely in areas that border the sea, where carbon steel, copper, and stainless steel can all cause galvanic corrosion with aluminum materials.
The following are some of the main factors influencing galvanic corrosion rates:
- The potential difference between materials.
- Cathode efficiency.
- Surface areas of connected materials (area ratio).
- The electrical resistance of the connection between the materials and of the electrolyte.
How to Prevent Galvanic Corrosion Through Design
Manufacturers who want to prevent corrosion through design with galvanic corrosion need to consider where and how the custom aluminum extrusions will be used. Due to the natural corrosion resistance of aluminum, galvanic corrosion will not occur in dry, indoor temperatures and rural atmospheres are not conducive to it either. When different metals are used in combination, galvanic corrosion can still be prevented through electrical insulation that breaks all contact between the metals. Paint can be used to prevent an electrolytic bridge from forming between the metals in large constructions where insulating would be difficult.
Pitting is a type of corrosion that occurs in materials that have protective films, it most often occurs as local damage to the aluminum surface and usually results in aesthetic damage rather than functional damage. It is an attack with localized holes on the metal’s surface. The attack can penetrate the metal very rapidly, while some parts of the metal surface remain free from corrosion. However, it can only occur where an electrolyte, such as water or moisture, is present, which contains dissolved salts or chlorides.
The corrosion generally appears as extremely small pits or holes. These pits, in the open air, will in most cases only reach a minor fraction of the metal’s thickness and hence not affect the strength of the material. Pitting has a local character and is randomly distributed over the surface. The size and depths of the pits have a stochastic distribution over the attacked surface. In the open air, these pits are just a small fraction of the overall thickness of the metal, however underground in water or soil, penetration of pitting will be thicker. For example, it occurs in common dirt and debris as well as in environments where the water cannot be led away from the metal.
Pitting corrosion can be controlled by:
- Use of a more resistant material
- Ensuring that the fluids in contact with the material are either washed away or are injected at a high velocity
- Reducing the medium’s aggressiveness
- Use of cathodic protection
- Avoiding stagnant zones
- Use of appropriate materials for service conditions
- Proper use of inhibitors or control of fluid chemistry
- Use of a coating that will prevent pitting on metal surfaces
- The ability to maintain the protective film of the same material
How to Prevent Pitting Corrosion Through Design
Because pitting is usually so minimal, it is more of an aesthetic issue than a functional problem. Pitting is more frequent and severe on aluminum that has not been treated, so surface treatment through anodizing, painting, or coating will counteract pitting and prevent corrosion through design. Cleaning treated aluminum regularly just by rinsing it with water should help maintain corrosion protection. Never use alkaline detergents to clean treated aluminum. Creating custom aluminum extrusion with design profiles that will dry quickly and easily to prevent a build-up of alkaline materials will also help to prevent corrosion through design.
Crevice corrosion is the type of corrosion that can occur in the narrow crevices of custom aluminum extrusions, it refers to the attack of metal surfaces by a stagnant solution in crevices, for example around the edges of nuts and rivet heads. When dust, sand, and other corrosive substances are deposited on surfaces, they create an environment where water will accumulate and corrode the part. It can happen between two metals or between a metal and a nonmetal. This causes damage to the metallic part, which is initiated by the concentration gradient in chemicals.
The likelihood of crevice corrosion occurring in well-designed custom aluminum extrusions is very small, however, in marine atmospheres or on vehicles, water can collect in the crevices between aluminum surfaces, leading to superficial corrosion known as water staining. The water source is condensation, which can form when cold materials are taken into a warm location. Simply the change between night and day can cause crevice condensation to occur when the aluminum is stored outdoors.
Factors that influence crevice corrosion
- The type of crevice: either metal to metal or metal to nonmetal
- The geometry of the crevice: including the size of the gap, its depth, and the surface roughness
- The composition of the metal: the structure of the alloy composition can be Cr, Mo, or others.
- The environment: the pH, halide ions, temperature, and oxygen1
- The resistance of a material to crevice corrosion can be ranked and evaluated by its critical crevice temperature (CCT), but this has to be in accordance with the ASTM Standard G48-03. CCT is the minimum temperature in °C that can produce a crevice attack and is found to be lower than the critical pitting temperature (CPT).
How can crevice corrosion be prevented?
- Replace riveted joints with welded butt joints.
- Eliminate crevices in lap joints through continuous welding and soldering.
- Drain existing solutions on surfaces and avoid creating stagnant conditions.
- Use solid and non-absorbent gaskets.
- Use higher alloys.
How to Prevent Creviceg Corrosion Through Design
Sealing compounds and double-sided tapes can be used before joining together two components to prevent water from getting into the gaps. This is a simple and easy approach to prevent corrosion through design for custom aluminum extrusions. Taking it a step further, screws and rivets can sometimes be replaced with adhesive bonding to counteract the formation of crevices in custom aluminum extrusions.