Section 4 - The Book of Metal
"Make sure you are hard and that you last a long time!"
Just like mom always said, “it’s what’s on the inside that counts!” What your bolt is made out of really matters if you want it to last a long time. Most of the bolts you see on the shelf at the local hardware store, are not going to make it more than just a couple seasons. And you can’t just buy whatever you want on any bolt-specific online retailer, even if they just serve climbers.
Chapter 1 - Zinc
Iron ore is mixed with carbon and processed into steel which is the most common metal used on earth. Fun fact - there are over 3,500 different grades of steel!(1) If steel is left exposed to air and water, it will rust. Painting steel, like on cars and bridges, slows the corrosion process down, but paint is not practical in many applications as it doesn’t last very long.
So the next level of protection is to use chemicals and electricity to apply a very thin metal coating to protect it. Zinc can corrode up to 100x slower than other metals, so steel is often “zinc-plated”. Fun fact - zinc isn’t a hard metal, in fact it is less than half as hard as steel (159DPM hardness vs 70DPM hardness). The zinc is a “sacrificial coating”, so when it is plated on steel, it will always tarnish and corrode first. However it is very thin, and naturally doesn’t give long term protection in any environment with moisture. Plated steel is generally intended for interior uses.
To make steel last longer, more zinc can be added. However that takes a completely different process called galvanization. Hot-dipped galvanized coatings is a 7 step process creating a metallurgical bond and can achieve a bond of 3,600 psi (harder than the base steel). This creates the rough surface you see on galvanized nails, but since galvanized screws can’t be too rough, it is spun in a centrifuge to clear the threads of excess zinc, though is still requires an oversized galvanized nut.
Think of cooking a piece of chicken in a pan with a little oil in the bottom (zinc plating) vs deep frying that turkey (galvanizing). They both have oil on it, but one has a much thicker coating. Zinc plated products are not intended to be an outdoor building material, but galvanized is, however it doesn’t last forever and is not an ideal highline anchor. Plated steel bolts can last as little as 3 months in areas like Thailand, Brazil or Hawaii before they can be broken off by hand.
Chapter 2 - Stainless
The word “stainless” is thrown around like it is a type of steel, when in fact there are 5 types or categories with a total of 150 grades(2). Chromium and nickel are the 2 major ingredients to make steel LESS corrosive (not corrosive proof). They don’t plate steel with these metals, they melt them together creating an alloy. The two different grades you will see in climbing bolts are 304 and 316 stainless.
304 Stainless Steel is also referred to as 18/8 (18% chromium, 8% nickel). Most stainless climbing bolts and hangers are made out of 304 grade and are significantly more resistant than any plated steel but fail quickly when near the ocean.
316 stainless or “marine-grade” is 18% Chromium & 10% Nickel & 2% Molybdenum, and less than 1% of carbon, phosphorus, sulfur, nitrogen. The Molybdenum is added to help resist corrosion to chlorides (salts) like in coastal areas. Fixe sells marine grade wedge bolts, glue ins, anchors and hangers. 316SS is the most corrosion resistant mechanical bolt that you can buy since titanium isn’t available as a mechanical bolt, only glue ins. However, in those harsh conditions of Thailand, Brazil and Hawaii, 316 stainless climbing bolts can completely fail within 3 years and so something even more corrosion resistant is required.
Duplex stainless or PLX stainless or HCR (high corrosion resistant) or 904SS or steel grade 1.4462, whatever the hell you want to call it, is coming onto the scene as a super stainless option. Fixe sells this as a more corrosion resistant version of stainless, however they did have a recall on them because they were rusting, go figure! Petzl sells a HCR wedge bolt with a HCR hanger for the low low price of a what a car costs. Bolt Products in Germany have their “Sea Water” series with twisted rod glue ins that supposedly break at 100kn and last 50 years for around €10 each. Here is some toilet reading if you think PLX HCR is interesting. However, titanium shines (metaphorically more than literally) over stainless.
Chapter 3 - Titanium
With a tensile strength similar to alloy steel, almost half the density of steel (56%) and platinum level of corrosion resistance, it is the “Cadillac” of all bolts. It is estimated that they can last up to 200 years (see www.titanclimbing.com). Fun fact: titanium is the 9th most plentiful element on earth and melts at 3,135F (400F more than steel). Titan Climbing manufactured the first certified titanium glue in bolt. It’s a “P” shape (one continuous rod) so there is no chance for a weld point to break. It requires a 14mm, or ⅝ inch hole and holds 35kn. They are about 30% more expensive as marine grade stainless and similarly priced to Bolt-Products duplex SS, but last much much longer.
Chapter 4 - Galvanic Corrosion
Metals are finicky in that you can’t just mix any 2 that you want. They all have a different electric current and the metal with less nobility (less electrode potential) will corrode very rapidly if mixed with a higher nobility (more electrode potential). So if you mix a stainless steel bolt with a zinc plated nut or washer, the nut or washer will corrode quickly. If you mix a SS hanger with a zinc plated bolt, you won’t see the corrosion happening in the hole. And SS bolts with zinc plated hangers will be real obvious. This also includes galvanized chain links on stainless bolts. So be mindful of your bolt, washer, and hangers. They all need to be the made of the same metal and that metal should be at least 304 stainless if not better. See these photos as examples.
Chapter 5 - Stress Corrosion Cracking
We love the coast, but the coast doesn’t love our bolts. So many coastal areas, especially with, but not limited to, limestone eat away stainless bolts quickly, even 316SS. Fixe calls their 316SS “marine grade” but it is not suitable for all marine environments. See the bolts failures in these photos and see how important titanium glue in bolts are in coastal environments.
So titanium is the best option in any areas that have a risk of SCC. It may feel more expensive but it isn’t that much more. If a 100 year cost for an anchor is considered, titanium is significantly cheaper. This isn’t the solution to every area for every highline, mostly because it’s not practical to drill a 14mm hole by hand and some areas are too hot for glues to be effective. However, there are no other options for high corrosion areas and Titan Eterna bolt’s are great for most areas in general.
Chapter 6 - Conclusions
Never use zinc plated bolts. Just the slightest scratch and the iron underneath is exposed to corrosion. And think about what holds the bolt… the wedge or sleeve at the base of the bolt. That contact point is what holds everything, and now that contact point is compromised as it is scraped against the inside of the hole. Also consider that most highlining anchors are placed on TOP of rocks, allowing water to go into these holes and just sit inside, so it is very important that a bolt can withstand corrosion.
Don’t mix metals or you risk bimetallic corrosion, speeding up the corrosion of 1 of the components of your bolt. And coastal areas cause excessive exposure to corrosion that even 316SS or even PLX HCR stainless cannot withstand. Don’t be cheap with people’s lives and install the highest quality bolts for your highlines.