Esteemed listers:
On the subject of acid cleaning, most of the caveats that have been posted are true, or at least are based on a seed of truth. Hydrogen embrittlement most often occurs in a caustic environment, such as in a boiler drum being treated with caustic soda for PH control. However, it can also occur in a strong acid environment as well, given the right conditions. The pressurized environment drives the hydrogen into the steel matrix, and failure results in what is called "intergranular cracking". Steel is a solid "solution" of iron, iron carbides, carbon, and alloying elements. All of these are admixed in a molten state. When the batch freezes, the metals and other substances present all crystallize, in a controlled formation that has the desired properties. Not all the alloying elements or matrix metals do this perfectly, and the crystal sizes vary somewhat from place to place. A good way to look at it is that the music plays while the metal is molten, and when the music stops, or freezes, everyone rushes to get t
o a chair.Everyone who doesn't get to a chair in time gets left out of the mix. Since the crystals of metal and carbon and meta
lloids form the bond that gives the metal its strength and toughness, anything that interferes with this bond makes it weaker. The weakest points are always at the junctions of the adjoining crytals. When you break a piece of metal by fatiguing it, as in a spring that has flexed too many times, the granular appearance of the broken ends moves us to say that the metal "crystallized". Of course, this is not what happened at all. We know that ALL solid metal is crystallized. Since the weak points are the joints between the crystals, metal will always break along these cracks or weak points. This is what we see when we look at a failed piece. We see the ends of the crystals, which let loose from each other.
What is important to this discussion is that the same thing can happen without the metal being bent back and forth. For example, if you chrome plate a landing gear strut for an aircraft, and you fail to heat treat it properly afterwards to drive out the trapped hydrogen that was driven in by plating, it can fail spontaneously. That is, you walk out one day and the strut has just snapped of itself. The hydrogen, in trying to escape, has severed the bond between the crystals, and caused intergranular cracking. Loosely, this is known as hydrogen embrittlement, and can be very destructive.
Any process which exposes a steel or other metal surface to free hydrogen or radicals which are part hydrogen, can and will cause the steel to fail in this way. Locomotive boilers once exploded with surprising devastation from just this cause. One incident during WWII involved landing gear on fighters. They had been hard chrome plated, before the process was well understood. A lot of them failed by themselves, and cost the manufacturer a pile to replace.
It's true that a crankshaft will suffer from this problem, if acid dipped or tank treated in strong caustic. The degree of damage depends on the temperature, the pressure, and the structure of the steel itself. Since the damage is hard to detect, I suggest a different process for cleaning rusty cranks and rods, and other similar parts. Most large machine shops that process engines have a machine called a Steel- abrator, or similar name. This guy is like a bakery oven with rotating racks inside. Your parts are fixed on the racks, then the racks rotate inside the "oven". A blast of mild steel shot is introduced, and every nook and cranny is cleaned to bare steel. No damage is done, and the part emerges like new. This is done before any machining, and it can save a crankshaft that has rusted way beyond conventional machine practices. I just had a 230 crank treated thusly, and I can assure you it looks like new.
This is not a be all end all solution to cleaning parts, but I thought it might interest some.
As always, best regards!
CJ
This archive was generated by hypermail 2b29 : Fri Dec 07 2001 - 00:36:25 PST