Galvanic Corrosion Considerations for PV Arrays
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Photovoltaic modules are designed for an operational life span of more than 25 years in the field. The design of the entire installation, not just these expensive components, should target a similar life span. The focus here is to describe the potential impact of galvanic corrosion on array structures and to identify best practices to minimize this impact. While some visible cosmetic corrosion effects over time may be tolerable, the failure of structural components is clearly not.
The Galvanic Series
Galvanic corrosion is the result of an electrochemical reaction. For galvanic corrosion to take place, four things must exist simultaneously: an anode, a cathode, an electrolyte and a conductive path between two pieces of metal. A galvanic circuit is created in which the anode loses electrons to the cathode with the assistance of the electrolyte. The galvanic cell created by two dissimilar metals and the presence of the electrolyte operates only in one direction. Consequently, the anode eventually disintegrates.
In PV installations, the anode and cathode consist of metals, such as stainless steel, copper and aluminum. Water commonly serves as the electrolyte. Whether galvanic corrosion is a serious problem depends on the potential failure point. For a PV installation, the long-term effects of corrosion can range from unsightly finishes to racking or fastener failure.
The more dissimilar the metals, as reflected by their relative position in the galvanic series (see Table 1), the greater the corrosion potential in the galvanic circuit. The general rule is to avoid joining metals far apart in the galvanic series. For example, steel is anodic next to brass, and stainless steel is cathodic next to zinc or aluminum. Another way to read this is that steel corrodes next to brass and stainless steel, while aluminum and zinc corrode next to steel when an electrolyte and a conductive path are present.
Every metal has a standard electrical potential (voltage) based on its ability to release or accept electrons when in contact with a dissimilar metal and an electrolyte. In reality, the galvanic system is more dynamic than most published material on voltage-potential data suggests. The actual reaction that takes place between two metals in the environment is dependent on electrolyte concentration, pH, temperature and other factors. Rob Haddock from Metal Roof Innovations, manufacturer of the S-5! mounting clamp, provides a word of caution about using the galvanic scale. “Some installers might want to use the galvanic scale to identify dissimilar metals, but the graphical galvanic scale is not always a good way to determine whether one metal is compatible with another,” he says. “When metals oxidize, the oxide layer created is a new material that may or may not exhibit the electrochemical characteristics of the parent material.”
In general, the greater the potential between two metals, the greater the driving force of the galvanic circuit and the more rapid the corrosion rate. If the potential is small, the driving force may be of no consequence. Whether or not a metal serves as an anode or a cathode is determined by the neighboring materials. The local environment also influences the reaction.
Moisture provides the electrolyte that enables galvanic corrosion to occur. Generally speaking, as humidity increases, so does the rate of corrosion. Atmospheric contaminants, such as chlorides (in marine environments) and sulfur dioxide and nitrous oxides (in industrial locations), are deposited on array structures. Once deposited, the contaminants react with oxygen and water and typically increase corrosion rates by releasing electrons from the metal’s surface. It is interesting to note that corrosion rates can be effectively decreased in areas of high rainfall, as contaminants are regularly washed away from the structural materials.
Corrosion Mitigation Guidelines
The Advanced Materials, Manufacturing and Testing Information Analysis Center’s Guide to Corrosion Prevention and Control (download at ammtiac.alionscience.com/pdf/Corrosion_Hdbk_S2.pdf ) suggests some general best practices.
- Use only one material to fabricate electrically isolated systems or components where practical.
- If mixed-metal systems are used, select combinations of metals as close together as possible in the galvanic series, or select metals that are galvanically compatible.
- Avoid the unfavorable area effect of a small anode and large cathode. Small parts or critical components such as fasteners should be the more noble metal.
- Insulate dissimilar metals wherever practical, such as when using a gasket. It is important to insulate completely if possible.
- Apply coatings with caution. Keep the coatings in good repair, particularly the one on the anodic material.
- Avoid threaded joints for materials far apart in the series.
- Design for the use of readily replaceable anodic parts, or make them thicker for longer life.
How does this translate to practice? Since an anode, a cathode, an electrolyte and a conductive path are necessary to create a galvanic cell, controlling those four elements can decrease the rate of corrosion.