Compressed air safety

[H Pearce]

When I Googled, "what corrodes Stainless Steel" I could not find any conditions which you suggested are a severe potential problem. It would be a benefit to all members reading this thread if you could tell us what conditions you believe cause Stainless Steel to corrode so badly and unpredictably that it should not be used as a pressure vessel for air compressors.

Hi,
Sure, the 2 main issues I have encountered are crevice corrosion and inter granular corrosion. Details of both can be found online. Apparently they are relatively rare….. although I have come across them multiple times. My favourite ones were 2 year old pipe work that could be removed by hand with little force as all the material beside the weld had disappeared! Most likely caused by incorrect welding procedures. The other being a nice looking shaft, about an inch in diameter and a foot long. Looked fine except for a couple of scabby lumps of rust on the surface. Removing these revealed massive holes and an almost hollow shaft. Both these examples were in more extreme uses but have reinforced to me that SS cannot be trusted. Have never seen anything beyond simple corrosion on basic steel although I’m sure it can happen.

 

[Steamchick]

Accelerated corrosion tests used NaCl. I.E. common salt. Accelerated corrosion because the temperature, humidity and salt spray were cycled.
Cabinet was made from corrosion resistant stainless steel and was OK. Lamp fittings , etc. were less corrosion resistant Stainless.
Electrolytic corrosion occurs from electro-potentials from differing metals. Boats made of plastic are saved from that. Steel boats have zinc anodes attached for sacrificial corrosion. Either way the props don't corrode rapidly. Other stuff corrodes first. And are made from a stainless steel chosen for corrosion resistance. - I guess?
K2

 

[Steamchick]

I am only reporting experiences of rusty stainless, because it costs much more money to have better corrosion resistance. The operative word is resistance. Not elimination.
K2

 

[ajoeiam]

snip
A Doctor of Civil Engineering plagiarised this into "Concretions containing Calcium products in an acid water medium corrode and fail (aka ageing) and this can be described in 3 words. "Wet concrete corrodes". - Think of that one next time you are in a steel framed building or concrete building in an Earthquake, or driving at high speed beneath or over old concrete bridges on the highway, - or don't. Because the corroded brake lines on your 15 year old truck are more likely to get you. (There is water in the brake fluid that attacks the inside of the brake pipes!).
snip

Very interesting - - - would you care to comment on the concrete build and prolifically used by the Romans that doesn't corrode even in salt water?

 

[Badhippie]

I work with stainless steel everyday 309,316 and harden stainless. We see corrosion everyday. Citric acid is something that will cause corrosion. But one of the biggest problems as was mentioned above is the wrong fasteners being used. But high concentrations of salt and hot water and so many other chemicals will cause stainless to corrode. Everyone thinks stainless is the best thing since sliced bread. To be honest it’s not it has its places to be used just like everything else. But to think stainless won’t corrode or to give the false impression that it won’t corrode it does corrode we see it everyday. But it does take more then just water to corrode stainless

 

[Badhippie]

I also forgot to mentioned that another reason for corrosion In stainless. Is the use of the wrong type of stainless for the application. there are so many different types of stainless it’s hard to keep track of them all.

 

[krypto]

When I Googled, "what corrodes Stainless Steel" I could not find any conditions which you suggested are a severe potential problem. It would be a benefit to all members reading this thread if you could tell us what conditions you believe cause Stainless Steel to corrode so badly and unpredictably that it should not be used as a pressure vessel for air compressors.

McMaster-Carr is a major industrial supplier for the USA. While they usually aren't the cheapest, they stock a LOT of items and they also have an extremely good website. They also sell various styles of ASME air tanks including stainless steel tanks:

https://www.mcmaster.com/products/pressure-tanks/asme-code-compressed-air-storage-tanks-6/
First off, the 316 stainless steel tanks are 5X the cost of a normal steel tanks and this effectively ends the consideration of their usage as pressure cylinders for most people. The tight-wads in this forum would never spend the money!

More interestingly, the ASME-Code stainless tanks that McM carries can not be sold in Canada. What you need to find out is why Canada doesn't allow them.

 

[Steamchick]

Sorry, I wasn't around when the Romans made concrete. I only repeat what I remember that I was taught. (I.E. a small fraction!).
K2

 

[Badhippie]

There is no industry recognized definition for a marine grade stainless steel, even though the phrase is routinely used by many end-users. Chloride ions can cause localized corrosive attack (pitting and crevice corrosion) of susceptible stainless steels.[4]In a marine environment it must be made clear as to whether the stainless steel is submerged in seawater, or is simply near enough to the seashore such that it can be attacked by the chloride present in seawater by splashing or carried by onshore breeze.

When the stainless steel will be submerged, a pitting resistance equivalent number greater than 40 is typically specified as the minimum for resistance to seawater. Stainless steels, such as super austenitic stainless steels (for example UNS S31254 or N08367), or super duplex stainless steels (for example UNS S32760 or S32750) meet this requirement.[5][6]

Near the seashore 316L is typically considered the minimum grade for use in such a marine environment.[7]

Concentrations of chloride in seawater can vary, and splash zones can cause concentrations to increase dramatically by evaporation, thus the corrosive severity of marine environments can vary. The resistance of any stainless steel near the seashore will also be dependent on whether the stainless surface can be rinsed by rainfall, which will reduce the tendency for surface chlorides to concentrate by evaporation. Thus, the underside of overhangs will be more susceptible to corrosion due to lack of rinsing, also desert locations near the seashore will typically be more corrosive than seashores in locations with high rainfall.

 

[ajoeiam]

Sorry, I wasn't around when the Romans made concrete. I only repeat what I remember that I was taught. (I.E. a small fraction!).
K2

(Snickering) - - - hadn't thought so.
Ran into an interesting article about how roman concrete was still viable where our contemporary stuff had far less longevity - - - especially in salt water.
Can't remember where I saw the article - - - I might have to dig it up - - - wonder - - - would it be considered too far OT for here?

 

[TonyM]

When I designed trolleys for heavy load use in autoclaves(about 30 years ago) I specified austenitic stainless because the environment contained traces of chlorine. I recollect at the time there were instances of roof tension wires catastrophically failing in swimming pools because the martensitic wire ropes reacted with chlorine vapours causing stress corrosion cracking.

 

[Toymaker]

Getting back on topic: I still hold the opinion that air compressor tanks made from Stainless are far superior to tanks made from soft steel, and somewhat superior to aluminum tanks. I rank normal soft steel tanks in last place because it's not a question of IF the tank will rust through, it's a matter of when will it rust through, and will the rusted areas lead to a rather nasty bang. I rank aluminum tanks in second place as although they may develop pitting, they're not likely to corrode so badly as to cause the tank walls to become thinner, as happens to soft steel tanks with rust scaling.

Considering normal shop conditions, IMHO, Stainless makes the best tank. As has been pointed out, stainless is susceptible to a short list of environmental conditions (ie Chlorine), but, soft steel will be equally affected,...and how likely is it your shop will see high concentrations of air-borne Chlorine.

 

[GreenTwin]

One member mentioned that he extended the drain line at the bottom of the tank out perhaps 12", to give the condensate somewhere to go beside in the bottom of the tank.

I leave the drain valve open on the bottom of my tank when I am not using it, hoping that will help.

.

 

[Toymaker]

Why not coat the inside of the air tank with one of the many commercially available Fuel Tank sealants? Yes, depending on the tank's size, location, etc, etc, the actual coating process could be a royal pain in the arss, but a good sealant should prevent any and all rust and corrosion.

 

[H Pearce]

Why not coat the inside of the air tank with one of the many commercially available Fuel Tank sealants? Yes, depending on the tank's size, location, etc, etc, the actual coating process could be a royal pain in the arss, but a good sealant should prevent any and all rust and corrosion.

Thick layers of paint prevent visual inspection. Corrosion can be underneath and not seen. I have had either a clear coat, thin layer of what is probably red lead or nothing in air receivers.

 

[Toymaker]

Thick layers of paint prevent visual inspection. Corrosion can be underneath and not seen. I have had either a clear coat, thin layer of what is probably red lead or nothing in air receivers.

Fuel tank sealants are not paint, as they remain flexible for life. Such sealants have been used to seal aircraft fuel tanks for decades and they maintain their air-tight, fuel-tight seal around rivets and metal joints despite airframe vibrations, wing flexing, thermal changes, etc. Virtually all home-built metal frame aircraft that use aluminum fuel tanks are required to use an approved sealant.

 

[Kerrin Galvin]

Hi guys,
Bit late to the conversation, a couple of years back I fitted a inter cooler with water trap to my compressor, best thing out!
Prior to that even relatively short run times resulted in water in the tank, when using the sand blaster I’d end up with water all the way thru to the gun, not helpful.
After watching a few YouTube videos I purchase a 16 pass oil cooler, and a got given a water trap, which turned out auto drains when the compressor depressure’s.
My compressor is fully enclosed, straight from the factory. I removed the compressor to tank line, & brought a pile of AN8 fittings & flexible stainless braded line…. As recommended by the shop to handle the temp. The cooler is mounted off the side of the enclosure & has 2 4” muffin fans temporarily sitting under the cooler….. must get that sorted … setup to blow air up, I get a temp drop from approx 70 - 75 degrees C in to the cooler to 30 - 33 degrees C out, given the my shop was 25 on the day I think is pretty good! The water trap knocks out pretty much all the water, to the point with extended running I got nothing from the drain. The added bonus, cool air takes up less volume so maybe I get more in the tank!

Cheers Kerrin

 

[H Pearce]

Fuel tank sealants are not paint, as they remain flexible for life. Such sealants have been used to seal aircraft fuel tanks for decades and they maintain their air-tight, fuel-tight seal around rivets and metal joints despite airframe vibrations, wing flexing, thermal changes, etc. Virtually all home-built metal frame aircraft that use aluminum fuel tanks are required to use an approved sealant.

Give it a go and let us know what happens. Maybe it will become the new standard.

 

[Badhippie]

I would not recommend putting any type of sealant inside an air tank. Reason I say this is there are teltail holes drilled in the OD of certified tanks they are drilled to min. Tank wall thickness. When the tank wall corrodes to the min wall thickness the tank will start leaking at these holes thus giving you a teltail sign the tank needs to be replaced. Also when a certified tank has lost 5% of the weight when the tank was new it should be replaced. The weight when the tank was new should be stamped on the cert tag on the tank.

 

[HMEL]

Getting back on topic: I still hold the opinion that air compressor tanks made from Stainless are far superior to tanks made from soft steel, and somewhat superior to aluminum tanks. I rank normal soft steel tanks in last place because it's not a question of IF the tank will rust through, it's a matter of when will it rust through, and will the rusted areas lead to a rather nasty bang. I rank aluminum tanks in second place as although they may develop pitting, they're not likely to corrode so badly as to cause the tank walls to become thinner, as happens to soft steel tanks with rust scaling.

Considering normal shop conditions, IMHO, Stainless makes the best tank. As has been pointed out, stainless is susceptible to a short list of environmental conditions (ie Chlorine), but, soft steel will be equally affected,...and how likely is it your shop will see high concentrations of air-borne Chlorine.

Stainless is a very good choice if price not an issue. The water condensed is pure water with little or no ions and is thus highly corrosive. Oxygen enhances that effect. The reason its difficult to sell these stainless tanks in Canada is due to its process regulations which vary from province to province. They may very well agree on the design considerations but they may require different inspection procedures and the paperwork can be obnoxious. Pretty much requires getting a Canadian Engineer involved in processing the paper work. I look at this way. My tank is still working and has been for over 20 years. When it goes probably will just buy another compressor because the mechanical stuff will be worn out too. And when it leaks it going to the scrap yard.

The other even more expensive option is carbon fiber. Its a corrosion problem and the only way around it is to install air dryers in the system before the receiver tank. But you have to have a good financial justification for doing so such as downstream instrumentation of a process that can not tolerate water.

Of all the pressure vessel failures I have witnessed I don't ever recall one being an air receiver probably because they are removed from service at the first sign of failure.