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--part1_2d.c910f15.284b94f1_boundary Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit The problem with using an acid on high strength steel is not any pitting you can see. The problem is a condition known as Hydrogen Embritalment. All metals form crystals as they solidify, when you subject a high strength steel (a hard steel) to an acid, the protons (H) of the acid will tend to follow the boundaries between these crystals and then sit there. Now if the protons just sat along these boundaries there would not be a problem but what they do is go looking for other free protons to form a Hydrogen atom (H2). the area taken up by H2 is 100 to 1000 times the area taken up by a single proton. So now we have all this H2 pushing against these crystal boundaries. With time, this pushing will form micro-cracks that will join to form much larger cracks until there is total failure of the part. In the aircraft industry all steel hardened to above 180 KSI (about and the standard may drop to 160KSI soon) has to be baked at 250F for 8 hours after any treatment involving an acid. This baking has to be performed within 8 hours of leaving the acid solution. After 8 hours the damage has been done so baking after that time will not do any good. I do not know how hard an old steel 72 is, and it is most likely less then 180KSI, but if the tanks are hard then they can be in danger of exploding without warning. You would have to contact the manufacturer or do a hardness check to find out. Now if you had done an acid cleaning on the newer high pressure steel tanks, I would say you just scrapped them By the way I have seen high strength steel parts break in a plating bath and have also investigated aircraft parts that cost lives do to this type of embritalment. It is a real problem and can happen with any acid contact. Peter Johnson Sr. Project Manufacturing Engineer Material and Processes Sikorsky Aircraft --part1_2d.c910f15.284b94f1_boundary Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: 7bit <HTML><FONT FACE=arial,helvetica><FONT SIZE=2>The problem with using an acid on high strength steel is not any pitting you <BR>can see. The problem is a condition known as Hydrogen Embritalment. All <BR>metals form crystals as they solidify, when you subject a high strength steel <BR>(a hard steel) to an acid, the protons (H) of the acid will tend to follow <BR>the boundaries between these crystals and then sit there. Now if the protons <BR>just sat along these boundaries there would not be a problem but what they do <BR>is go looking for other free protons to form a Hydrogen atom (H2). the area <BR>taken up by H2 is 100 to 1000 times the area taken up by a single proton. <BR> <BR>So now we have all this H2 pushing against these crystal boundaries. With <BR>time, this pushing will form micro-cracks that will join to form much larger <BR>cracks until there is total failure of the part. In the aircraft industry all <BR>steel hardened to above 180 KSI (about and the standard may drop to 160KSI <BR>soon) has to be baked at 250F for 8 hours after any treatment involving an <BR>acid. This baking has to be performed within 8 hours of leaving the acid <BR>solution. After 8 hours the damage has been done so baking after that time <BR>will not do any good. <BR> <BR>I do not know how hard an old steel 72 is, and it is most likely less then <BR>180KSI, but if the tanks are hard then they can be in danger of exploding <BR>without warning. You would have to contact the manufacturer or do a hardness <BR>check to find out. <BR> <BR>Now if you had done an acid cleaning on the newer high pressure steel tanks, <BR>I would say you just scrapped them <BR> <BR>By the way I have seen high strength steel parts break in a plating bath and <BR>have also investigated aircraft parts that cost lives do to this type of <BR>embritalment. It is a real problem and can happen with any acid contact. <BR> <BR>Peter Johnson <BR>Sr. Project Manufacturing Engineer <BR>Material and Processes <BR>Sikorsky Aircraft</FONT></HTML> --part1_2d.c910f15.284b94f1_boundary-- -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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