Galvanic Corrosion: When Different Metals Meet

What Is Galvanic Corrosion?

You installed a brand-new stainless steel pipeline in your chemical plant, connecting it with brass fittings and copper bolts. Everything looked perfect at the start. But just six months later, you noticed the copper components had turned green, the brass fittings were cracked, and worst of all - the expensive stainless steel pipe had developed strange pits near the connection points.

What went wrong? The answer is a phenomenon called galvanic corrosion - one of the most common and costly forms of material degradation in industrial settings. It occurs whenever two different metals are electrically connected and immersed in a conductive solution (such as seawater, acidic water, or even humid air). In simple terms: when two different metals touch each other and get wet, one metal starts to lose electrons to the other. The metal that loses electrons is called the anode - it corrodes. The metal that gains electrons is called the cathode - it is protected.

This process is named after Luigi Galvani, an Italian physicist who discovered the phenomenon in the 1780s. While studying frog legs twitching with electricity, he inadvertently revealed one of the most important principles in electrochemistry that engineers still rely on today.

Galvanic corrosion (also called bimetallic corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another metal in the presence of an electrolyte. The more active metal (higher tendency to corrode) becomes the anode and corrodes faster than it would alone. The less active metal becomes the cathode and corrodes slower - or not at all.

Galvanic corrosion is especially dangerous because it can cause rapid, localized damage. While general corrosion spreads evenly across a surface, galvanic corrosion attacks one specific area - the junction between two metals - with concentrated intensity. A joint that should last 20 years might fail within 2 years if the wrong metals are paired. Industries most affected include:

Marine and offshore engineering (seawater is an excellent electrolyte)

Chemical and petrochemical processing plants

Power generation and desalination plants

Architecture and construction (dissimilar metal roofing and fastening systems)

Oil and gas pipelines and storage systems

The Galvanic Series: A Ranking of Metal Activity

The Galvanic Series (also called the EMF series or galvanic compatibility chart) ranks metals and alloys based on their electrochemical potential in seawater. Metals at the top (more negative potential) are anodic (will corrode). Metals at the bottom (more positive potential) are cathodic (will be protected). The greater the distance between two metals in the series, the faster the galvanic corrosion will occur when they are coupled.

Here is a simplified galvanic series relevant to the stainless steel and nickel alloy industry:

Table 1: Simplified Galvanic Series in Seawater (Saturated Calomel Electrode Reference) - Source: ASTM G82 and NACE Corrosion Data

Practical Rule: When selecting metal pairings, choose metals that are adjacent in the galvanic series (within 100 mV of each other) to minimize galvanic corrosion risk. A distance of more than 300 mV indicates a high-risk combination.

Four Key Factors That Accelerate Galvanic Corrosion

The Role of the Electrolyte

The electrolyte is the liquid or moisture that conducts ions between the anode and cathode. The more conductive the electrolyte, the faster galvanic corrosion proceeds. Seawater is the most aggressive electrolyte - its high salinity (approximately 3.5% salt) makes it an excellent conductor. This is why marine environments are the most demanding for dissimilar metal connections. Other common electrolytes include: freshwater (moderate conductivity), acidic solutions (highly conductive and corrosive), humid air (forms thin electrolyte film), and concrete (alkaline pore water accelerates embedded steel corrosion).

Surface Area Ratio

The relative surface areas of the anode and cathode matter enormously. A small anode coupled with a large cathode is a dangerous combination. Why? Because the same amount of corrosion current is distributed over a smaller anode area, causing more concentrated and rapid damage.

Conversely, a large anode with a small cathode spreads the corrosion over a larger area, reducing the damage rate. Example: a small stainless steel screw (cathode) threaded into a large aluminum plate (anode) causes rapid aluminum corrosion at the thread interface. But a large zinc sacrificial anode attached to a steel structure corrodes slowly and protects the steel for years.

Temperature

Higher temperatures generally accelerate electrochemical reactions. As temperature increases, the conductivity of water increases, ions move faster, and corrosion rates climb. However, there is a notable exception: in some passivated stainless steel systems, corrosion rate can decrease at very high temperatures due to changes in the oxide layer stability. Always consult the material data sheet for temperature-specific behavior.

Mechanical Design and Microbial Influence

Poor mechanical design can trap moisture and create crevices where electrolyte accumulates. These crevice corrosion zones become self-perpetuating - the electrolyte inside becomes more concentrated and corrosive over time. Additionally, microbiologically influenced corrosion (MIC) can accelerate galvanic attack when bacteria colonies form in crevices, producing localized acidic environments.

Case Studies in the Metals Industry

The Marine Heat Exchanger Failure

A coastal desalination plant used titanium tubes in a copper-nickel (Cu-Ni) alloy shell. Although both materials were chosen for their seawater resistance, a galvanic couple formed between titanium and Cu-Ni. Within 18 months, the Cu-Ni shell developed severe grooving corrosion at the tube sheet interface. Root cause: titanium is significantly more cathodic than Cu-Ni in seawater (+200 mV vs +50 mV), creating a strong galvanic driving force. Solution: installing a non-conductive gasket between the tube sheet and the shell broke the electrical circuit and eliminated the galvanic couple.

The Chemical Storage Tank Incident

A chemical facility used a 316L stainless steel tank to store hydrochloric acid (HCl) solution. The tank was equipped with carbon steel support legs. The acidic electrolyte from HCl solution bridged the connection between carbon steel (anode) and 316L stainless steel (cathode). The carbon steel legs corroded through in just 8 months, creating a critical safety hazard. Solution: replacing carbon steel legs with 316L stainless steel brackets and adding a protective coating to isolate any remaining carbon steel components resolved the issue.

The Offshore Pipeline Story

An offshore oil platform used 254 SMO super austenitic stainless steel flanges connected with Hastelloy C-276 bolts in a seawater service environment. At first glance, both are highly corrosion-resistant alloys. However, 254 SMO sits higher in the galvanic series than Hastelloy C-276, making 254 SMO the anode. The Hastelloy C-276 bolts (cathode) were protected - but the expensive 254 SMO flanges suffered selective corrosion at the bolt hole interfaces. Solution: switching to Alloy 625 bolting (galvanically closer to 254 SMO) stopped the corrosion and extended the service life by an estimated 15 years.

How to Prevent Galvanic Corrosion?

Material Selection Strategy

The most effective prevention strategy begins at the design stage. Use the following decision framework:

Table 2: Galvanic Risk Assessment and Action Guidelines - Based on NACE SP0177 and ASTM G82 standards

Insulation and Isolation

Installing non-conductive materials between dissimilar metals is one of the most effective and economical prevention methods. Common solutions include:

Thermoplastic washers (nylon, PTFE/Teflon) between flanges

Rubber gaskets at pipe joints

Non-conductive pipe hangers and clamps

Epoxy or polymer coatings on one or both metal surfaces

Dielectric unions in piping systems

Cathodic Protection

For buried or submerged structures, cathodic protection is the industry-standard solution. Two approaches are used:

Sacrificial anode system: Attach a more active metal (zinc, magnesium, or aluminum alloy) to the structure. The sacrificial anode corrodes instead of the protected structure. Common for offshore platforms, water tanks, and underground pipelines.

Impressed current system: Use an external power source to apply a protective DC current to the structure, shifting it to a cathodic (protected) state. Used for large structures like ships' hulls and large-diameter pipelines.

Design Principles

Good design can eliminate galvanic corrosion without any special materials. Key principles:

Avoid small anodes adjacent to large cathodes - use large sacrificial anodes or balance surface areas.

Design for drainage: Ensure water cannot pool at metal joints.

Allow for inspection: Do not bury dissimilar metal joints in inaccessible locations.

Use same-family materials whenever possible: stainless steel bolts with stainless steel flanges.

Apply marine-grade coatings (epoxy, polyurethane, or thermal spray aluminum) to carbon steel components in marine environments.

Material Pairing Guide for Stainless Steel and Nickel Alloy Users

For professionals working with stainless steel and nickel alloy products, the following table provides practical pairing guidance based on industry experience and corrosion data:

Table 3: Material Pairing Guide for Industrial Applications - For use as general guidance; always consult material specialist for specific service conditions

Summary

Table 4: Key Takeaways from This Article

Conclusion

Galvanic corrosion is a predictable, preventable, and manageable phenomenon. It does not have to be a mystery or an unavoidable hazard. By understanding the galvanic series, knowing your environment, applying smart design principles, and selecting compatible material combinations, you can significantly extend the service life of your equipment and reduce costly failures.

At Jinie Technology (Jiangsu) Co., Ltd., we have over 15 years of experience helping customers select the right materials for demanding applications. Our technical team understands the nuances of galvanic compatibility in seawater, acidic, and high-temperature environments. Whether you need 316L stainless steel piping, Hastelloy C276 fittings, or super duplex flanges, we provide not just the products - but the expertise to ensure they perform reliably in your specific service conditions.

If you have a specific material challenge or need technical guidance on material selection for a corrosive environment, our engineers are ready to help.

Contact Us: Market@jnalloy.com | +86 1933 990 0211 | www.jnalloys.com