Avoiding Building & Structure Galvanic Corrosion

Atmospheric, crevice and galvanic corrosion are all common causes for metal deterioration in buildings and structures. Galvanic corrosion is often misunderstood. This article focuses on avoidance of galvanic corrosion for non-immersed applications on buildings and structures. In general, if a metal would corrode in a given environment on its own, then a galvanic connection to a more noble metal will usually increase the corrosion rate of the less noble metal and will slow the corrosion rate of the more noble metal.

The Statue of Liberty may be the most famous and well-studied galvanic corrosion failure. Despite initial separation of the cast iron framing and copper, the cast iron began corroding quickly as the inert barrier failed. Smaller armature replacements began not long after construction and the first large-scale replacement of the iron armature and the original stairs occurred between 1937 and 1938.  An evaluation done in advance of the 1976 US centennial celebration made it clear that a major restoration effort was required.  The eventual replacement of most of the iron with stainless steel between 1981 and 1986 is described in an earlier issue of Stainless Solutions, a MolyReview article, and on IMOA’s Structural Restoration page.

Requirements for Galvanic Corrosion
When a metal is in contact with a conductive liquid (electrolyte), it has an electrical or corrosion potential. This potential is determined by the anodic and cathodic reactions on the surface and is measurable. When two metals are connected, the electrons will flow from the electronegative (anode) to the electropositive (cathode) metal, increasing the corrosion rate of the anode. Three elements must be present for galvanic corrosion to occur:

1) Two or more metals with different corrosion potentials.
2) Direct contact between the metals.
3) A conductive electrolyte connects the metals (e.g. water from rain, fog, or condensation).

The presence of salt and some pollutants increases the conductivity of the electrolyte and the corrosion rate, so assessment of the environment is important. The structural supports for rain screens and sunscreens are exposed to the environment and condensation is common within walls and under roofs. It is therefore important to assess the inherent atmospheric corrosion performance of any metal in a specific environment first and then to consider the possibility of accelerated corrosion due to a galvanic connection.

Galvanic Series
The Galvanic Series in Seawater for Some Metals and Alloys

The “galvanic series” in seawater is used to predict whether this type of corrosion is a concern when metals and alloys are in direct contact and moisture is present. The most anodic metals are at the top and will actively corrode to protect the more cathodic (noble) metals further down the list. It is important to look at both placement in the series and the corrosion potential values. Larger value differences imply a stronger electrical current and increased anodic metal corrosion rates. However, it is not possible to accurately predict the increased corrosion rate based solely on these numerical differences.

Furthermore, buildings and structures are complex. Some sections may have increased exposure to moisture, salt, or pollutants or not benefit from rain or manual cleaning and may be more vulnerable than others.

Relative Surface Area
The relative surface area (not mass) of each of the metals is also an important factor. If the area of the cathode (more noble metal, electropositive) is larger than the anode (less noble metal, electronegative), the electrical current produced is likely to be stronger and the rate of corrosion of the anode will be faster than for equal surface areas. This effect increases with larger differences in surface area.

The copper industry advises the use of austenitic stainless steel (i.e. 304/304L, 316/316L) fasteners and structural support components for copper installations, because the metals are relatively close in the galvanic series and the coefficient of thermal expansion is equivalent.  Furthermore, stainless steel is more noble and, when used for smaller components, is unlikely to have any significant galvanic effect on the larger copper surface. Stainless steel fasteners are regularly used for other less noble metals.

While galvanic corrosion can lead to failure of the less noble metal, it is also used to extend the life of more noble metals. For example, coatings containing zinc and aluminum are applied to carbon steel because those metals are more anodic and will corrode first, extending the life of the steel.

Design Considerations
If it is not possible to avoid unfavorable dissimilar metal combinations, the best solution is to electrically insulate one from the other. Paint, inert washers and other methods are used. Their expected service life should be assessed relative to the design life of the project and inspections considered.  Abrasion caused by differences in the metals’ coefficients of thermal expansion can accelerate inert barrier deterioration.  When painted carbon steel and stainless steel are welded together, the welded joint and some of the adjacent stainless steel should be painted.

Dissimilar metal connections are a particular concern when the more anodic metal is structural and has a smaller surface area. If contact occurs at any location, then all of the continuously connected metals are part of the galvanic cell and determine the relative surface area ratio. If a few inert washers between aluminum or carbon steel framing and stainless steel panels are missing, if paint abrades due to differences in coefficients of thermal expansion, or if metal burs cut through thin double-sided adhesive tape, then electrical contact at those connections can weaken the structure as was shown in the Cathedral example.  Specifying fasteners with built-in inert washers and similar precautions can reduce problems but may not be a permanent solution.

Stainless steel roofing or façade elements should be supported by stainless steel structural sections, fasteners and clips to achieve maximum service life. Additionally, it is also important to make sure that fasteners connecting multiple metals match the most noble metal.

General guidance for locations with moderate salt exposure and similar surface areas is given in the table. Large differences in surface area ratio will change the corrosion rates. For example, a stainless steel bolt in a large carbon steel plate in a rain shedding application is generally not a concern, but the railing example above shows that the opposite significantly increases corrosion rates. The appropriateness of metal combinations and the risk of corrosion will vary with the specific environmental conditions and design details.

Guidance on the Risk of Galvanic Corrosion in a Marine or Deicing Salt Exposed Atmosphere for Some Common Metals with Similar Surface Areas

Some Exceptions & Anomalies
Galvanic corrosion can occur without initial physical contact. For example, localized corrosion of a metal may result in water soluble corrosion products which can deposit on to the surface of another metal.  This can cause localized, intense galvanic cells. This is a particular concern with more electropositive metals like copper and it is why downspouts and gutters from a copper roof should always be in copper or stainless steel and not aluminum.  Always consider the relative placement of metals on a building.

Not all environments produce the same galvanic results and using the galvanic series for seawater may not always provide reliable predictions. For example, research has shown that galvanic corrosion is not a concern between stainless and carbon steel in concrete. In this alkaline environment, there is more galvanic corrosion between sections of carbon steel that are active (i.e. corroding) and passive (i.e. not corroding) than between stainless and carbon steel.

Other Resources
The Nickel Institute brochure, Guidelines for Corrosion Prevention, provides information on every common type of corrosion and includes data on the relative corrosion rates of metals in different environments.  IMOA has a Stainless Steel Selection System with more specific guidance on evaluating exterior service environments and selection of appropriate alloy(s) to avoid atmospheric corrosion. There are also specific pages on avoiding corrosion caused by soil, coastal and deicing salt exposure.

Corrosion engineering is a specialized area within metallurgical engineering and materials science.  It can be quite complicated and experts specialize in specific environments such as corrosive water, industrial environments or atmospheric corrosion. If there is uncertainty, the advice of an expert is recommended.