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Posted: 9/2/2009 8:46:01 AM EDT
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I have a stainless AR barrel that I want to have blasted to give it a matte finish. When I contacted a local 'smith about doing it, he replied that I probably shouldn't. He specifically said:
"I haven't had very good results bead blasting the stainless barrels. They will rust if there is the slightest amount of impurities in the blast materials so I haven't been blasting stainless much. If you maintain your barrel it isn't an issue but if it sits for a long time or gets wet it could be." I have a couple other stainless barrels that I bought already blasted and I haven't had any issues with them, but now I'm wondering if I should get this one done or not.
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Quoted:
I guess I'm assuming that he means that any impurities in the blasting media (iron/carbon steel particles?) might get embedded in the stainless. The stainless won't rust per se, but the embedded particles will?
I thought SS had very little iron in it, so how would blasting it cause rust? BTW I know SS will rust over time and saltwater. I asked him for clarification, but am awaiting his response. |
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Stainless steel has LOTS of iron in it; just like any steel it is an iron-based alloy. Stainless NEVER equaled rustproof.
That said, it depends on what grade of stainless we're discussing (304 and 440C have VERY different corrosion resistance properties). Also, it depends on what sort of finish was present from the manufacturer. Bead blasting just MIGHT make the corrosion situation worse in certain cases. As to the point about cleanliness of blasting media - this is VERY IMPORTANT. You can actually passivate stainless steel by blasting it with virgin aluminum oxide grit. But if there is some non-stainless bits in the blasting media, from previously blasting non-stainless steels and then recycling the media, then you are INVITING pitting corrosion on your part. And yes - I AM a metallurgist and I have personally passivated stainless steel aerospace parts. |
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Quoted:
Stainless steel has LOTS of iron in it; just like any steel it is an iron-based alloy. Stainless NEVER equaled rustproof. Exactly, in normal conditions stainless will take a long time to rust. The problem with bead blasting is that you're not only increasing the surface area of the steel, but you're also making small pits that will hold moisture, that will give it plenty of surface and time to react. Just take some minor precautions, keep the surface oiled so moisture can't get in, don't leave it wet, make sure your long term storage is dehumidified. Remember, when air cools, humidity goes up, so don't just store it in a soft case under the bed |
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I know that Chris Reeve Knives says that a stainless steel blade that has the satin finish or received their "stone wash" treatment is more corrosion resistant than blades that are beadblasted.
http://www.chrisreeve.com/faq.htm#blades Same may hold true for stainless rifle barrels. |
| As mentioned above, don't use recycled blasting material that was used on non-stainless steel and you"ll be fine. I myself blast more carbon steel than SS so when I do SS I use new material and spray it outside, open-air (not in a cabinet). I have a 2nd cabinet that I am setting up for SS but haven't done it yet. When someone blasts parts in a cabinet the blast media is re-used, along with the removed particles of whatever was blasted. So if you used the blast cabinet on carbon steel and then SS you are essentially blasting carbon steel onto the surface of the SS and that's what rusts. This is also the reason to NOT use regular steel wool on guns, steel wool gets left behind or embedded and IT will rust on top of whatever you used it one. |
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