Aluminum is an important type of metal that is used in many structures. Despite the fact that it has less strength, it has some benefits due to its flexibility and lack of weight in comparison with other metals. In some cases, aluminum and steel are used together. For example, the use of stainless steel screws in aluminum is common. However, this type of connection can create some problems due to galvanic corrosion. Corrosion is a common problem in different process industries. In this article, you will find ways to contain this problem and safely perform a connection like this one.

Can Stainless Steel and Aluminum Be Used Together?

There is no doubt in it that stainless steel is the best option for holding the aluminum and other light-weighted metals together. Aluminum is usually used in the form of sheets and high strength fasteners are required for holding these sheets together. However, there are many dissimilarities in aluminum and stainless steel. But when the environment is dry and sheltered, there are fewer chances of the galvanic corrosion. Stainless steel and aluminum can be used together but in most cases, both metals catch corrosion. There can be several reasons behind the corrosion and they will be discussed in the next sections of the article. 

An interesting fact about the assembly of stainless steel and aluminum is that stainless steel fasteners can hold the aluminum sheets but the aluminum fasteners cannot hold the stainless steel components. The best example of the safe installation of stainless steel screws in aluminum is the parapet guards used in the sideways of the bridges and roads. 

What Happens When Stainless Steel and Aluminum are Used Together? 

Whenever the dissimilar metals come in contact, the chances of galvanic corrosion arise. The same is the case with aluminum and stainless steel. 

When those metals come in contact, galvanic corrosion occurs. Actually, the electrons begin to transfer from one metal (anode) to the other metal (cathode). Apart from the two dissimilar metals, there is a need for an electrolyte that provides an appropriate environment for galvanic corrosion. In the case of aluminum and stainless steel, the moisture in the air or the water works as an electrolyte. The transfer speed of electrons depends on the level of resistance of the electrolyte. More will be the resistance, lesser will be the transfer rate. 

When it comes to aluminum and stainless steel, aluminum reacts as an anode when the second metal is stainless steel. The relative surface area between the aluminum and the stainless steel is very important. Corrosion of the aluminum begins when stainless steel generates corrosion currents with a high concentration on the sacrificed piece of aluminum. Stainless steel bolts exhibit the required strength but they corrode the areas around them if they are fastened in aluminum sheets. Corrosion between aluminum and stainless steel can begin in the following cases:

• The presence of a conductive liquid between the joining surface of the aluminum and stainless steel screws. 
• The aluminum sheets and the stainless steel screws are directly in contact. 
• The potential difference between the metals is sufficient that electrons of one metal excite the electrons of the other metal. 

Apart from the corrosion, aluminum also affects the color of stainless steel. The discoloration of the stainless steel screws occurs because of their direct contact with the aluminum sheets. 

When does the Galvanic Corrosion between Aluminum and Stainless Steel occur?

The previous section describes the three conditions that can cause corrosion. This section will provide a detailed overview of the galvanic corrosion. The following are the three important reasons behind the occurrence of galvanic corrosion.

Humidity in the Joint

In the areas of the high humidity, moisture in the air can act as an electrolyte. However, under normal conditions, rainwater or water coming from other sources can cause corrosion. The extent of the corrosion depends upon the conductivity of the electrolyte. Water coming from the process industries has high concentrations of salt and salt increases the conductivity of the water. Moreover, during the evaporation process, the conductivity of the water films increase, and the galvanic effects of water increase also. 

Metal to Metal Contact

Metal to metal contact is not harmful in normal conditions. However, the electrical contact can cause severe galvanic effects. Actually, during the contact of two dissimilar metals, one metal acts as a noble metal and it does not corrode. Similarly, during the contact of the aluminum and the stainless steel, stainless steel acts as a noble metal and it only corrodes the aluminum. There are very few cases in which the stainless steel corrodes because of the aluminum. The use of an electrically insulated coating on stainless steel can help in preventing this type of corrosion. 

Potential Difference

The conductive liquids have the capability of dissolving metals. Again, the extent of the dissolution of the metals into the conductive liquids depends upon the conductivity of the liquids. The corrosion current generates when the contact surface area of the noble metal (stainless steel) with the conductive liquid is more than the contact surface area of the sacrificed metal (aluminum). Galvanic corrosion occurs when the potential difference between the two metals is more than 0.1 volts. The proper insulation of the stainless steel screws reduces the potential difference by restricting the electrical contact. 

How to Prevent Galvanic/Bimetallic Corrosion?

After understanding that what happens when the aluminum and the stainless steel are used together and the details of the galvanic corrosion, this section will highlight the practices that can help in preventing the galvanic corrosion. The following points are very helpful for the prevention of corrosion:

• In practical situations, the introduction of the electrically insulating materials between the aluminum and stainless steel reduces the chances of galvanic corrosion. Different types of coatings and paints serve the purpose of electrical insulation. A modern technique is to use high-density primers that can cut down the electrical path between the metals.  
• If the complete insulation of both metals is possible, gaskets and other insulating materials are very helpful for completely insulating them. The use of spacers between the two dissimilar metals also reduces the chances of corrosion. 
• Working another way round by controlling the surrounding environment also helps in reducing the chances of corrosion. Different inhibitors can reduce the aggressiveness of the environment. The environment plays an important role in the corrosion of metals as well as in the prevention of corrosion. In such cases, the primer is applied to the metal with high susceptibility to corrosion. 
• The use of replaceable anodic parts can also help in reducing galvanic corrosion. Moreover, increasing the thickness of the anodic parts ensures a long life for the stainless steel and aluminum assembly. 
• The use of a metal that is anodic to aluminum and stainless steel in the galvanic contact with the sacrificial metal proves to be very helpful for stopping corrosion. 
• The insulations and regular maintenance of the parts are very helpful in preventing the stainless steel screws from discoloration. 
• Passive films are very helpful in corrosion resistance. Usually, they cover the fasteners. It is very important to cover the cathode (stainless steel screws) with insulations otherwise the corrosion could be worsened. 

Apart from the aluminum sheets and stainless steel screws, the metals that are close enough in the galvanic series are very stable against the galvanic corrosion. 

How to Safely Use Stainless Steel Screws in Aluminum?

This section highlights the important steps that can help in the safe use of stainless steel screws in aluminum. 

Do Not Let Water in

As discussed before, water is very harmful to the stainless steel screws. Marine water is very harmful to the aluminum-steel assembly. Coatings, paints, or tapes are used for reducing the contact point between the metals and the water. The direct contact of aluminum and stainless steel is not harmful and it does not cause corrosion unless there is some conductive medium such as electricity or water. So, the protection of connecting points between the stainless steel screws and aluminum ensures the prevention of corrosion. The removal of an electrical conductor by preventing the metals against the water makes the stainless steel and aluminum relatively inert. In the absence of water, direct contact between the aluminum sheets and stainless steel screws is not harmful. 

Check the Surface Area

It is very important to make sure that the piece of aluminum has almost 10 times larger surface area than the stainless steel screw. The introduction of a non-reactive barrier between the metals reduces the chances of galvanic corrosion. The production of the galvanic current increases with the increase in the reduction of oxygen. And the reduction of oxygen increases when the cathodic part is larger than the anodic part. 

Installing a Sacrificial Anode

The introduction of the third metal that is anodic to both stainless steel screws and aluminum ensures the prevention of corrosion. The sacrificial metal is more anodic than the parent metals and it sacrifices itself by getting corroded. While the third metal acts as an anode, the current flows through it because the protected metal starts acting as a cathode. Relatively pure active metals act as sacrificial anodes such as magnesium and zinc. Specially developed anodes of magnesium alloys and aluminum alloys are used in modern applications. The most fundamental applications of the sacrificial anodes are water heaters, underground tanks, refineries, hulls of ships, and pipelines. 

Leaving margin for Corrosion

 Another option for the safe operations of the stainless steel screws is to leave some margin in aluminum for the corrosion. A slight increase in the dimensions of the aluminum near the stainless bolts will let the extra aluminum to corrode. 

Conclusion

Corrosion can create huge problems in metallic structures. After, everything that you have written in this article you believe that the environment can create problems with this connection maybe you should look for other ways to do it. Especially, if human safety is at stake you should be very careful. 

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