"Make sure it is hard and that it lasts 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 market directly to climbers. Sometimes they sell interior products without them clearly labeled as such (ahem… Fixe).
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! 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 and will wear off when there is constant metal to metal contact like in climbing.
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 the threads on galvanized screws can’t be too rough, it is spun in a centrifuge to clear the threads of excess zinc, though it 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 outdoor 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. Chromium and nickel are the 2 major ingredients to make steel more resistant to corrosion (not corrosion 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. 316SS is the most corrosion resistant mechanical bolt that you can buy since titanium isn’t available as a mechanical bolt (only as glue ins). However, in the 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 1.4362/1.4462, whatever the hell you want to call it, is coming onto the scene as a super stainless option. It is roughly 50% ferritic steel and 50% austenitic steel making it twice as strong as either ferritic or austenitic by themselves. 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! They narrowed down the batches affected and manufacture the hangers differently now. We currently haven’t found any reports of them failing outside of those batch numbers and we really like how they perform (strength-wise) in BoltBuster tests.
However, Peter Randelzhofer put out a paper testing Fixe’s anchor PLX chain links that were in an outdoor covered climbing gym in the Netherlands and they discovered cracks near the welds. Apparently duplex steel 1.4362 is easier to weld but duplex steel 1.4462 is better for corrosion. So the chains they tested were 5kn under the 25kn mbs in only 2 years in mild outdoor conditions.
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.
Fun fact: INOX is sometimes stamped into bolts. It is a french way of saying stainless from the word “inoxydable”. It could be 304 or 316 SS but it doesn’t mean duplex stainless, that has PLX stamped into it.
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 abundant 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 made from one continuous rod so there is no structural weld point to break. It requires a 14mm, or ⅝ inch hole and has an MBS of 35kn. Our BoltBuster tests all were above 35kn in shear and in tension. They are about 30% more expensive than marine grade stainless and similarly priced to Bolt-Products duplex SS, but are much more corrosion resistant.
Chapter 4 - Durability
Sometimes, corrosion isn’t the #1 concern but wear and tear. Jim Titt from www.bolt-products.com demonstrated in an experiment that titanium wears down about twice as fast as stainless does. He buried these metal links in a box with dry, sandy soil and had a 10mm rope run back and forth 1.6m or 5 feet through the two different metals and got these results…
0 cycles - SS 8mm, Titanium 8mm
100 cycles - SS 7.52mm, Titanium 7.07mm
200 cycles - SS 6.76mm, Titanium 5.74mm
300 cycles - SS 6.07mm, Titanium 4.37mm
400 cycles - SS 5.53mm, Titanium 3.57mm
Titanium (top photo) wears about twice as fast as SS (bottom photo)
Chapter 5 - 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 made of the same metal and that metal should be at least 304 stainless if not better.
See these photos as examples.
Chapter 6 - Stress Corrosion Cracking
We love the coast, but the coast doesn’t love our bolts. So many coastal areas, especially in Thailand or Malta, eat away stainless bolts quickly, even 316SS. Fixe calls their 316SS “marine grade” but it is not suitable for all marine environments. Other factors that speed up corrosion can be elevated temperatures, crevices, mixed metals, mixture of high and low humidity, overhanging rock where rain cannot rinse off harmful chemicals and compounds. See the bolts failures in these photos and see how important titanium glue in bolts are in corrosive environments. This video is also gnarly!
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 if it doesn't have to be replaced. High traffic areas will handle wear and tear better with stainless steel components so you need to weigh the risk of corrosion with the frequency of ropes running over the metal.
Chapter 7 - Staining
Just like skid marks on your underwear, we should avoid the same problem on our rocks. When the zinc coating on plated steel is exposed to hydrogen and oxygen (aka water) long enough it creates zinc hydroxide (similar to iron oxide which is rust). Zinc hydroxide is a white powder that forms and can leave streaks on the rock. Also when the iron is exposed, it will rust, not only making the bolt dangerous but leaving the areas looking… shitty.
Chapter 8 - Camouflage
Ideally, bolts would only be seen from the last clip in point and not be shiny bling you can see from space. In an effort to keep our climbing areas less impacted, people have tried camouflaging bolts and hangers. The biggest problem with this is that it doesn’t last very long, especially as people clip carabiners to them and that metal on metal contact wears down any effort someone put in.
PAINT - Roughing up the metal with sandpaper and spraying them with Rustoleum at home prior to installation is one way but that process doesn’t last forever if the bolt is exposed to elements or used often. You can increase the adhesion by priming it first and doing several THIN layers. Etching zinc plated hangers I hope is obviously bad as the zinc coating is super thin and when you paint wears off, you have unprotected steel exposed. Plus zinc or galvanized metal cannot have an oil based (alkyd) product on them. The process of the galvanized layer and the alkyd creating a layer of soap is called saponification and shit starts peeling like your skin with a bad sunburn. Try painting galvanized gutters with oil based paint… I can tell you from experience as a painter… it doesn’t work! If for some weird reason you care enough to install zinc plated shit but still care enough to camo them, please use water based primers and products.
Stainless isn’t stoked for some etching either. Jim Titt from Bolt-Products.com explains the risk in painting stainless in this forum “Stainless steel gains it´s corrosion resistance by producing chromium oxide which is passive and prevents further surface corrosion by blocking oxygen diffusion to the steel surface, this blocks corrosion from spreading into the metal's internal structure. Passivation occurs only if the proportion of chromium is high enough and oxygen is present, a coating which prevents oxygen reaching the surface prevents passivation from happening. The usual problem is that the coating either becomes damaged (by tightening the bolt or by karabiners) or porous due to ageing and allows water to penetrate which becomes anaerobic. We passivate all our products during manufacture and any attempt to apply a surface coating using methods such as abrading or etch priming is removing the passive layer.”
Alternative Coatings - Powder coated definitely lasts longer than paint and Metolius and Fixe have some available but selection is very limited. Automotive paint can be a fancy and expensive way to camo.
Plasma coatings are next level in overkill if you really are determined to hide your bolts while having chalk tick marks on the route! I doubt plasma will be available from your climbing bolt suppliers anytime soon when people try to save pennies per hanger when establishing routes.
Heat treating - This seems to be a fancy permanent way to mess up, I mean color, your bolts. How much can you heat up steel, stainless or duplex steel before it is compromised? If you don’t know, don’t do it but if you want to nerd out on it go to this forum on Mountain Project. And please don’t blow torch a glue in that is already installed! The glue doesn’t like that. Supposedly ClimbTech will darken wave bolts for you if requested.
Chapter 9 - 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 may not withstand and so titanium might be your best bet. If it is a lower off anchor and will have ropes running through it constantly, stainless will hold up twice as long as titanium. Don’t be cheap with people’s lives and install the highest quality bolts on your climbing routes or for your highlines.
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