Compatibility of stainless steel with other metals | Galvanic couple

The compatibility of stainless steel with other metals depends on the specific type/grade of stainless steel and the metal it comes into contact with. Stainless steel is compatible with some metals, but there are some that when combined (in contact) form a galvanic pair, which can lead to rust and subsequent corrosion (bimetallic | galvanic | contact) of your product.

Having two dissimilar metals in a structure is not always a sign of a problem, but it can be a problem. When two metals are in direct contact, there is the potential for a bimetallic pair to form. There are four elements required for contact metals to corrode:

1. One of the metals must act as an anode and generate electrons that can create an electric current flow.
2. Another metal must act as a cathode and collect these electrons. This metal is the protected partner of the corrosion cell.
3. There must be an electrolyte covering the two metals where they touch to complete the path of the electric current. This electrolyte material must be able to produce ions from one metal to the other.
4. There must be a return path for the current, which in all cases is direct contact between the two metals.

The existence of two dissimilar metals in direct contact cannot be a problem unless there is an electrolytic material.

In most atmospheric applications, the only potential electrolyte material that may be available is rainwater or dew. Both of these forms of water are poor electrolytes because they do not contain many salts and ions that would make them leading. On the other hand, marine environments and areas where melting snow contains road salts can be very good electrolyte materials. Bimetallic pairs are easier to form when immersed, when the unit is submerged during operation. Salt water is especially aggressive to two dissimilar metals in contact.

The "Galvanic Series Table" shows metals and their electrochemical voltage range (relative activity in running seawater). A typical rule of thumb is that a voltage difference of 0.2 volts or more indicates a risk of galvanic corrosion. In general, the further apart the metals are in the galvanic series, the greater the risk of corrosion, with the least noble (anodic) grade metal usually corroding.

It should be noted that although stainless steels are generally found in the more noble or cathodic part, studies show that stainless steels exhibit a different range of stress in crevices or stagnant and poorly aerated water (these values ​​are given in brackets).

There are several factors to consider when choosing stainless steel, as they affect its compatibility with other metals and alloys:

• Stainless steel type: different types of stainless steel have different chemical compositions, which affects their resistance to corrosion and compatibility with other metals.
• Other metal / alloy: different metals and alloys have different chemical properties that affect their compatibility with stainless steel. Some metals, such as copper and aluminum, can corrode stainless steel under certain conditions.
• Conditions: the conditions in which stainless steel is used also affect its compatibility with other metals. Temperature, pressure, and the presence of chemicals can affect the rate of corrosion.

The galvanic series table (metals and their electrochemical voltage range) can be used as a quick guide to selecting compatible metals and stainless steel. Where two materials meet in the green zone, the corrosion potential can be considered negligible, i.e. a good match. However, materials in the red zone should be avoided or used with caution.

Remember: incorrect combination of stainless steel with other metals and alloys can lead to tragic consequences. Stainless steel is an alloy with high resistance to corrosion. However, when in contact with certain metals and alloys, its corrosion properties may deteriorate.

If the connection or contact between stainless steel and an incompatible metal or alloy cannot be avoided, try to protect the stainless steel from contact. Use insulating materials such as Teflon or epoxy resin, paint coatings. This will prevent direct contact between stainless steel and other metals and alloys.