Mixed Metals and Galvanic Corrosion: Ticking Time Bomb or Tempest in a Teapot?

It's a common situation in climbing: You reach the crux of a route only to find the bolt protecting the hard moves is rusty. How trustworthy is that bolt? Is this just surface rust or is there a more destructive process, galvanic corrosion, at work, making the bolt unsafe?

Galvanic corrosion and its effects on fixed climbing hardware are among the most misunderstood aspects of our sport. To many climbers, the utterance of the words "mixed metals" brings thoughts of self-destructing bolts and dangerous routes.

To understand what's happening with corrosion and fixed anchors we have to step back a bit in time. Nowadays, most climbers in the US have accepted that only matched metal components of stainless steel or titanium are acceptable for long-term use. But, for many years, it was common to have climbing anchor components—the bolt which goes into the rock and the hanger—be made of two different metals.

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Scores of routes had carbon steel or plated carbon steel bolts with stainless steel hangers. This is known as mixed metals. Metals corrode over time. The National Association of Corrosion Engineers, NACE International, has identified eight different forms of corrosion. To be sure, all eight forms of corrosion can affect climbing hardware, but the focus of this article is on one type, galvanic corrosion, because it occurs when two different types of metal come in contact with each other.

Galvanic corrosion is an electrochemical process with one metal being the anode and the other being the cathode. The anode is the more "active" metal. The more "noble" metal is the cathode.

The driving force causing the corrosion is the difference between how "active" the anode is and how "noble" the cathode is. Metallurgists use a table called a galvanic series to rank metals. The most "active" metals include zinc and magnesium while the most "noble" metals include platinum and gold.

The most commonly used metals in climbing are carbon steel, zinc-plated carbon steel, and stainless steel (304 and 316 series). Carbon steel and plated carbon steel are considered the same metal since once the zinc has sacrificed itself you are left with carbon steel. On the galvanic series list, carbon/plated steel is more "active" while stainless steel is more "noble."

What this means in the climbing world is that if you have mixed metals such as a carbon steel or plated carbon steel bolts with a stainless steel hangers, and galvanic corrosion does occur, the bolt will corrode while the hanger remains whole. This is why steel is plated with zinc, also known as galvanizing. Rather than the steel undergoing corrosion, the zinc, which is more "active" compared to steel, gives itself up first.

Several environmental factors can affect the driving force which causes galvanic corrosion, with water being the most significant because it provides a pathway, the ion path, for the movement of hydroxyl ions between the two dissimilar metals.

What's in the water also has an effect on the speed of the corrosion. Because the process of galvanic corrosion is the transfer of electrons from the anode to the cathode and the formation of chlorides and oxides or hydroxides, anything which enhances these two processes will also accelerate galvanic corrosion.

This is why salt water is so destructive. Salt is sodium chloride, and the chlorides in the salt water help with the formation of the iron chlorides. Also, water flushes away the iron chlorides which helps accelerate the corrosion. For this reason, mixed metals, and even stainless steel anchors are not recommended for climates which are regularly exposed to salt water (i.e. very near coastlines). Titanium has been proven to be the best metal for this type of climate.

Plain old water does not contain enough electrolytes (salts), such as the chlorides, to accelerate galvanic corrosion. Some dissolved minerals in water can actually retard galvanic corrosion. However, even though water does not significantly accelerate galvanic corrosion, being continuously exposed to water does facilitate the process, hence it is not the ideal situation for anchors with mixed metals. The Pacific Northwest is a good example of a high water environment. Mixed metal anchors in these areas should be replaced.

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In most environments in the United States, fixed anchors are not continuously exposed to water. For sure, it rains and snows, but it is not a constant condition. How are mixed metal anchors in these areas affected by galvanic corrosion? One of the problems with identifying the type and amount of corrosion is that it is very difficult to distinguish galvanic corrosion from plain rust and since most of the bolt is in the rock, one cannot obtain a complete view of the corrosion without removing the bolt.

Over the past five years I have replaced over 1000 bolts in Northern California and in Colorado. Working with the Department of Civil and Environmental Engineering at Colorado State University, a sample of the removed rusted carbon steel and plated steel bolts in a mixed metal configuration have been tested for strength in shear, which is the most common loading of bolts in vertical and less-than-vertical placements.

Many of these bolts were 30 years old and very rusty, but virtually all tested at, or very close to, the strength of brand-new bolts. This indicates that galvanic corrosion is not a hazard for these bolts—and most likely the areas where the bolts were removed for testing. This also indicates that while surface rust looks ugly, it does not significantly affect the strength of a bolt. These bolts were safe for climbing.

Another interesting observation is that, when a plated steel or carbon steel bolt is paired with a plated steel or carbon steel hanger, rusting occurs to the bolt even though the metals are the same. This indicates that rusting, not galvanic corrosion, is the most likely form of corrosion in climates where there is not continuous contact with water 

In summary, galvanic corrosion of mixed metal climbing anchors is a concern in areas where salt is present, such as sea cliff climbing. In areas such as the Pacific Northwest, where there may be continuous contact with water, galvanic corrosion is facilitated, but not necessarily accelerated as it is in salt water environments.

For all other climbing areas there is likely not enough continuous contact with water for galvanic corrosion to be a concern. Bolts in these areas may appear rusty, but it is most likely only surface rust. They are still safe. If you are concerned about the corrosion of bolts in your climbing area, removing a sample and working with professionals to inspect and test them will give you a good indication of the type of corrosion occurring, their safety, and the urgency of their replacement.

The following contributed to this article: Dr. Guna Selvadury, Professor and Chair, Department of Biomedical Engineering, San Jose State University; Dr. Paul Heyliger, Professor, Department of Civil and Environmental Engineering, Colorado State University-Fort Collins; Dr. Hussam Mahmoud, George T. Abel Professor in Infrastructure and Director, Structural Laboratory, Department of Civil and Environmental Engineering, Colorado State University-Fort Collins; Dan Merrick, P.E., Board Certified Forensic Engineer.