Fixing Corrosion Between Anodized Aluminum and Steel

Q: Why does corrosion occur between anodized aluminum and steel?

A: Aluminum is a reactive metal (unnoble) which should have a low corrosion resistance, according to thermodynamics. The high corrosion resistance found on aluminum, nevertheless, is due to the presence of a thin, compact film of adherent aluminum oxide on the surface. Whenever a fresh aluminum surface is created and exposed to either air or water, a surface film of aluminum oxide forms at once.

This aluminum oxide dissolves in some chemicals, notably strong acids and alkaline solutions. When the oxide film is removed, the metal corrodes rapidly by uniform dissolution. In general, the oxide film is stable over a pH range of about 4.0 to 9.0, but there are exceptions.

One of these exceptions is in environments where the surface film is insoluble, but weak spots in the oxide film lead to localized corrosion. Local corrosion can only be found when aluminum is passive, covered by an oxide layer as the one formed by anodizing.

Localized corrosion has an electrochemical nature and is caused by a difference in corrosion potential in a local cell formed by differences in or on the metal surface.

Galvanic corrosion is due to an electrical contact with a more noble metal or a nonmetallic conductor in a conductive environment. The galvanic corrosion is very dependent on the cathode reaction and which metals are in contact with each other.

The efficiency of this cathodic reaction will determine the corrosion rate. The most common examples of galvanic corrosion of aluminum alloys are when they are joined to steel or copper and exposed to a wet saline environment.

The galvanic corrosion of aluminum is usually mild, except in highly conductive media such as slated slush from road deicing salts, sea water and other salty electrolytes. The contact area must be wetted by an aqueous liquid or humidity in order to ensure ionic conduction. Otherwise, there will be no possibility of galvanic corrosion.

The galvanic series for metals show that aluminum will be the anode of the galvanic cell in contact with almost all other metals and, hence, the one which suffers from galvanic corrosion.

The galvanic corrosion performance of the different aluminum alloys is quite similar, so that changing alloys cannot solve the problem.

The dissolution rate depends on the surface ratio between the two metals: corrosion = cathodic surface area / anodic surface area. The most favorable case is a very large anodic surface area and a small cathodic surface area. The galvanic corrosion is a local corrosion and is, therefore, limited to the contact zone.

It is unusual to see galvanic corrosion on aluminum in contact with stainless steel (passive). In contrast, contact between copper, bronze, brass and different kinds of steel alloys (passive and active) and aluminum can cause severe corrosion, so it is advisable to provide insulation between the two metals.

The reason for the confusion if galvanic corrosion happens or not between aluminum and steel is that stainless steel can be found as passive or active, and if the environment contains chloride. This will substantially change the corrosion effect on aluminum.

Generally, the closer one metal is to another in the series, the more compatible they will be (for example, the galvanic effects will be minimal). Conversely, the farther one metal is from another, the greater the corrosion will be.

Thorough prediction of galvanic corrosion is not easy. For contact between common metals, in particular steel and stainless steel, experience shows that laboratory testing always leads to more severe results than what is actually observed under conditions of weathering.

Normally, the galvanic coupling with stainless steel works very well, but, when there is even the slightest trace of chloride in the environment, a galvanic corrosion will take place.

Due to the cracking of the anodized layer when mounting, a very little area of unnoble metal (the aluminum underneath) will be in contact with a very big area of the more noble metal (the stainless steel). This will cause galvanic corrosion, which can increase tremendously, depending on the area of the aluminum.

Q: Why is the sealing process so important?

A: Sealing is the last step in the anodizing process and can be done by several different processes, though the main reason for all the different sealing processes is to close the porous aluminum oxide layer after the anodizing step.

Without a high quality sealing, the anodic coating feels sticky and is highly absorbent to all kinds of dirt, grease, oil and stains. The sealing gives a maximum corrosion resistance, but minimizes the wear resistance of the anodized oxide layer.

The simplest process takes place in boiling, deionized water. Other solutions with a variety of additions of sealing salts can be applied. The most common ones are hot DI sealing, midtemperature sealing and cold sealing.

When Hot DI sealing, the anodized part is immersed into hot (96-100°C/205-212°F) deionized water and a hydrated aluminum oxide (boehmite) will be formed in the pores. The process begins by the precipitation of hydrated aluminum oxide as a gel of pseudoboehmite. This precipitation is controlled by diffusion, pH and chemical composition of the sealing solution.

Increasing pH will start a condensation of the gel and then crystalline pseudoboehmite forms and the pores will be filled up. During the last period of sealing, this pseudoboehmite will recrystallize to form boehmite starting at the surface. This hydrated aluminum oxide (boehmite) has a greater volume than the aluminum oxide.

Many different sealing times have been suggested, but the most common used in Europe is 2-3 minutes per µm of oxide layer. This process will happen partially by itself over time with the moisture in the air.

This process is very dependent on the temperature and pH of the sealing solution. Sealing at 96°C (210°F) requires about 6% longer sealing time than 98°C (210°F). This dependency of the temperature makes midtemperature sealing, which works at 60-80°C (60-80°F), a little more prone to leaching of colors. These solutions often contain metal salts and organic additives, but have a lower energy cost. The process is still using the fact that aluminum oxide is hydrated to boehmite.

Cold sealing uses a totally different mechanism than the other two. In this process, the sealing happens by an impregnation process at 25-30°C (70-90°F). The following is suggested to happen — the fluoride in the solution dissolves the porous anodized layer and then deposits as a fluoroaluminate at the top 3-6 µm (0.1 -0-2 mil) of the layer as “equation.” This process is very slow, so a warm water rinse after the sealing will accelerate the impregnation process.

Anne Deacon Juhl, Ph.D., is president of AluConsult. Visit aluconsult.com.