Hydrogen Embrittlement and BMX
Why Chrome is bad By Sheepdog

In BMX people have been clamoring for better/ lighter/ stronger parts.  Using aerospace alloys like Titanium,7075 aluminum, and more.  When using these materials, certain bits of caution must be used. While I had thought chrome parts had pretty much died, a few companies still use it on parts. Especially cranks.  I had long believed part of the reason chrome fell out of favor was its inherent problems, that was not the case.  It was only out of fashion.

Well, before chrome makes a comeback and you buy that shiny new set of chrome cranks, you may want to read this and get familiar with it. Also, you may wish to inform your favorite companies of the hazards of chroming certain parts. Since they seem as oblivious as the rest.

Before we start, I am no metallurgist (metals scientist), however this information is from Boeing, chrome facilities, and an automotive magazine who did have metallurgists. This is not new information, just not well known, or has been ignored. So if you have extremely in depth questions regarding how strong something will be, or was it this that broke that, etc... I am not going to be able to tell you. I can make guesstimates based on what I know, that's about all. My advice is the same as what you will read in this article. Stay away from chrome (except on rims).

Material Susceptibility chart published in Street Rodder magazine (I assume they got it from Boeing)

First place I heard about Hydrogen Embrittlement was in an article in Street Rodder magazine. Street rods have long used chrome in parts, and this was written to help educate people on the dangers of chroming every piece of driveline and suspension part. It blew me away how ignorant of it the BMX community was. Still is ignorant of it. For a group using the most susceptible metals and getting it from aerospace suppliers I expected more. What amazes me more, is it has been mentioned in hot rodding magazines since the early 70's.

One of the first quotes in the Street Rodder issue is a quote from Bill Snyder (a quality control manager at one of the largest military and civilian plating companies). "A hydrogen embrittlement failure is catastrophic [in this case, a sudden  and violent event]. Pow! I've seen aircraft-grade  steel bolts four inches in diameter shear right in half. Hydrogen embrittlement can occur in a part with a wall three inches thick if you have enough hydrogen disrupting the molecular structure of the metal. Hydrogen embrittlement won't occur unless a part is loaded, but under experimental conditions we've had internally loaded test bands with .25in. wall thickness explode during plating!" 

How it works...
Tensile strength is the pulling of a material until it tears apart. The higher the tensile strength, the more susceptible it is to embrittlement. When you chrome a part, hydrogen is soaked into the material as it is plated, the plating then holds it in. Once inside it will spread to all areas, where it is relatively harmless. When stress is applied, the hydrogen re-distributes itself, concentrating on the point of stress. Think of it as trying to escape, and its only way out, is a crack. Metallurgists do not know how a crack is formed. That is still in debate. The moving and concentration however,  is not. "Presently this phenomenon is not completely understood and hydrogen embrittlement detection, in particular, seems to be one of the most difficult aspects of the problem. Hydrogen embrittlement does not affect all metallic materials equally. The most vulnerable are high-strength steels, titanium alloys and aluminum alloys." 

When enough hydrogen accumulates, and a crack is established, it focuses on the end points of the crack. Until it becomes so weak that the end of the crack moves, the hydrogen then follows to the new highest stress point. The new end of the crack. Until the whole thing fails suddenly.

Low allow steels (LAS), like 4340, and 4130(chrome-molly) are the MOST susceptible material to this phenomenon. Strangest part of hydrogen embrittlement is that the stronger the material, the more likely it will fall victim. The new Supertherm, Platinum OX, and Reynolds tubing, is just modified 4130, with an even HIGHER tensile strength.

There is a fix...
Only its not pretty.. Literally. The most common method is to bake the parts. Baking a part at 375 +/- 25 deg. F. for 3 hours will usually correct most of it. Now, your thinking you can just put your frame into your oven..  Wrong.. It MUST be done within 1 hour of plating.  This only works if the plating is porous enough, and it will make the chrome turn bluish. Ever see chrome headers on a car...  Not to mention, since its decorative chrome, it will likely NOT be porous enough, and will drive the hydrogen deeper into the tubing. Making things worse.

Some platers do have special chrome baths they can use that reduce the amount of hydrogen introduced. unfortunately, that costs more than a bike frame would be worth. Even if it was cheap, how would you know if that is what the plater used.

So what do the experts recommend?
Don't chrome anything structural. No chrome forks, frames, bars, or cranks.

I have ridden chrome parts over the years. I have even broken a good portion of them. Did they seem any more weak than painted? I am not really sure, but riding styles change all the time, you never jump the same things the same way, etc.. So we have no real way to know based on just riding a part. I do know I have had better luck with painted parts as I got bigger and I do remember at least one frame that when it broke, fit the description of embrittlement.

Would I ride a chrome fork, or frame again... for flatland.  Would I go out jumping on a chrome S&M Black bike.. Not a chance

References
Street Rodder Magazine Oct. 1991
Boeing Aircraft Company
Corrosion-Doctors

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