Contact corrosion: Chemical affinity and its impact

Corrosion is much more than just superficial rust. When incompatible metals are joined, their direct contact can cause profound damage due to galvanic corrosion. The process behind galvanic corrosion is based on electrochemical processes and preferentially occurs in wet, salty, or aggressive environments. Under these conditions, it leads to the inevitable degradation of the less noble metal. In this article, we show how contact corrosion works, which materials are particularly affected and which preventive measures are effective.

A definition of contact corrosion

What is corrosion or contact corrosion? Contact corrosion, also called galvanic corrosion or bimetallic corrosion, is the result of a chemical reaction that occurs when two different metals with different electrochemical potentials (standard potentials) come into contact with each other. There is a measurable change in the metal, e.g. a color change or deterioration of the surface quality, up to and including degradation and complete destruction. You can find out more about the general corrosion mechanisms here.

In order for galvanic corrosion to occur, other conditions must be met in addition to the different standard potentials: The metals involved must be in an electrically conductive system, such as through direct contact or via conductive fasteners. Moist or salty environments are particularly corrosion-promoting because they act as electrolytes. An unfavorable surface area ratio, such as a large cathode in combination with a small anode, can significantly accelerate the corrosion process. A typical example is joining aluminum and stainless steel in a maritime environment where the less noble aluminum is heavily attacked.

Differences in corrosion susceptibility

Not all metals are equally susceptible to corrosion. The standard electrochemical potential, which indicates how strongly an atom attracts or emits electrons, is the key. The element hydrogen with a standard potential of E⁰ = 0 V serves as a reference value. Electrons are negatively charged elementary particles. Bound to the atomic nucleus, they form the so-called electron shell of an atom. Simplified, each atom has an atomic nucleus filled with positively charged protons, which attracts the negatively charged electrons located in the surrounding electron shells by a potential difference. In its pure form, each element has a certain number of these protons in the nucleus and a certain number of electrons per electron shell in the electron shell. The electrons located in the outermost electron shell are also called valence electrons. Valence electrons are the electrons that can react with other atoms in a chemical reaction.

Within the atom, the positively charged nucleus and the electrons in the electron shell face each other. If the positive charge of the nucleus outweighs the negative charge, a positive standard potential is created. The atom attempts to attract further electrons to equalize the potential and holds the already bound electrons and thus also the valence electrons of the outer shell particularly firmly in the electron shell.

Precious metals such as gold or silver have a positive standard potential (E⁰ > 0 V), therefore accept electrons and act as oxidizing agents. Metals with a standard potential (E⁰ < 0 V), such as base metals, also seek to equalize the potential but give off electrons. During this process, they are reduced, so they are a reducing agent.

The greater the difference in potentials between both metals and the better the electrolyte conducts, the more intense the REDOX (reduction-oxidation) reaction is. For this reason, galvanic corrosion is stronger with salt water than with fresh water. Therefore, when selecting different materials, care should be taken to ensure the lowest possible potential difference in order to minimize the formation of a galvanic cell and the associated material removal.

Some metals are particularly susceptible to corrosion, especially when used unprotected in humid or aggressive environments. These include steel, brass, zinc, and iron.

Other metals provide natural protection:

• Precious metals (e.g. gold, platinum) are resistant due to their electron configuration
• Passivating metals (e.g. aluminum, titanium) form stable oxide layers
• Copper and zinc develop protective layers from their corrosion products

Despite their durability, stainless steels, particularly austenitic stainless steel, can also corrode in unfavorable combinations (such as contact with zinc or aluminum). While a passive chromium oxide layer protects it, galvanic corrosion can also occur under certain conditions (see our article Machining of stainless steels).

What influences contact corrosion?

  In addition to the presence of moisture (an electrolyte) and general chemical compatibility, the intensity of corrosion or corrosion rate is also affected by:

• Further substances involved: Chloride or bromide ions, such as those found in salty air or detergents, accelerate corrosion processes by attacking the protective passive layers.
• Impurities: Residue such as weld spatter, abrasive dust, salt residue or residue from assembly aids (e. g. flux) can locally lead to potential shifts or gap formation. Read how LABS, for example, forms an attack surface on surfaces here.
• Surface ratio: An unfavorable surface ratio (small anode, large cathode) dramatically increases the current density on the anode surface, resulting in particularly intense local corrosion.
• Temperature: Higher temperatures increase electrolyte conductivity and accelerate reaction rates, resulting in faster corrosion.

Types of Contact Corrosion

Contact corrosion can take different forms depending on the material pairing and environmental conditions – from visible surface corrosion to locally limited damage such as spot corrosion or pitting, which are often particularly treacherous and difficult to detect.

Pitting corrosion is a localized form of corrosion. It manifests itself as small, often needle-like depressions or holes in the metal surface that can penetrate deep into the material. It is often triggered by chloride ions, which attack and undermine the protective passive layer of certain metals. In the resulting depressions, aggressive ions continue to accumulate, which significantly accelerates the destruction process locally. Since the affected areas are often covered by corrosion products such as rust, pitting corrosion remains unnoticed for a long time.

Another type of corrosion is gap corrosion. Gap corrosion occurs on metallic components when a corrosive medium accumulates in narrow, difficult-to-reach spaces such as overlaps, support surfaces or incompletely welded seams. The cause is a concentration gradient between the medium inside the gap and the outer region favored by the restricted diffusion of oxygen or other reaction partners in the gap. These concentration differences produce an electrochemical potential difference that leads to local corrosion either within the gap (hydrogen type) or in its immediate environment (oxygen type). Gap corrosion can be enhanced by contact corrosion

Avoiding contact corrosion in aluminum / stainless steel

First of all, the selection of suitable metals should be the focus. It may be possible to avoid using a particularly susceptible metal. As already mentioned, these include steel, iron, zinc, brass, but also aluminum and copper. Particularly for direct metal joints in humid or salty environments, any combination of these materials with more noble metals is problematic. The use of hot-dip galvanized steel instead of galvanized steel significantly improves corrosion protection, e.g. in combination with aluminum. The thicker zinc layer on hot-dip galvanized steel provides longer-lasting cathodic protection and delays exposure of the steel surface. This allows the risk of galvanic corrosion to be reduced for the long-term without significantly affecting the mechanical properties of the component. Special layers such as LTBC combine corrosion protection with glare protection. Details can be found here.

• Electrically insulate the metals from each other, e.g. by insulating intermediate layers. There must be no contact at all between the metals.
• Interrupt the electrolyte connection, e.g. by using an insulating layer or a special surface coating. It is recommended to coat the surface of the more noble metal. Examples of a suitable surface coating include: Paint, rubber, anodized layer, galvanically produced layers of gold, copper, etc. For more examples, see this article.
• Use a so-called sacrificial anode. An anode made of a less noble metal (often by galvanizing) is attached in direct metallic contact with the metal to be protected (e.g. aluminum), so that the less noble material primarily corrodes in a purposeful manner and thus protects the actual metal part against corrosion.

Protection against contact corrosion

The following illustrations show two options for engineered corrosion protection and illustrate how corrosion behavior changes when using an insulator and a coating.

In order to avoid galvanic corrosion, only metals and alloys with approximately the same standard potential should be paired. (see also: table of galvanic pairs at the end of this article)

The following overview shows galvanic pairs between selected metals and alloys: