Brass or SS Materials is Better?

Yes, brass alloy and stainless steel has good resistance to corrosion also use in marine application.

Never use brass in a marine environment. Silicon bronze but never brass.

Actually Admiralty and Naval Brass are both highly resistant to salt water corrosion due to the addition of Tin to the alloy, and Aluminium Brass can be even better under some conditions.

CDA 83600, which is 85-5-5-5, is a commonly used "leaded red brass", often called Bronze by many people, that was and still is used aboard many marine vessels and in many marine environments. Yellow Brass on the other hand, is not really suitable for use in marine environments. Depending on the valve size I have found that small valves are often die cast yellow brass while larger valves tend to be either 836-leaded red brass(composition bronze) or for higher temperatures Navy "M" (922) or "G" (903) bronze.

I would also agree with the concern about galvanic corrosion possibilities due to the difference in potentials.

You should never use "normal" Brass in a marine environment, it simply falls apart after a relatively short time period, weeks if not months......

Bronze is often used instead on ships and boats for such environments, but how good it is for valves in the application, I have no idea at all.......but someone here will know for sure!!

You can also evaluate the use of protection by coating or appropiate paint system.

http://www.carboline.com.tr/media/11842/water_wastewater_brochure_web.pdf

http://www.rematiptop.com/brochures/CORROSION-PROTECTION-Rubber-Lining-and-Coating-Systems.pdf

http://www.coleparmer.com/Chemical-Resistance

Yes,

You may use almost any valve material you want as long as it is rated for the pressure and is resistant to whatever you are transporting inside the pipe.

At 8 bar that is approx. 120 psi so a typical brass or stainless valve should have the rating of 200 WOG (Water - Oil - Gas). Since it looks like you are transporting compressed air, you have many choices. Look at the surrounding area, is it easily accessible? If it is in a difficult to get at area and change out the valve I would suggest the SS valve. It will cost more but you may not need to replace it as often as say a cheaper valve.

Good luck

Bryan

If using Brass in conjunction with 316 SS be aware of the potential (pun intended) for galvanic action to occur. There is roughly 200mv to 400mv difference in corrosion potential between the two alloys, the actual voltage will be dependent on whether the SS is active or passive under the conditions of use.

This is substantial, and will create corrosion problems under the correct (or incorrect) conditions, and could well result in the failure of the Brass fitting in a quite short time period which will depend on the aggressiveness of the electrolyte, with the more aggressive one generally producing a slightly longer lifespan due to the stripping of the passivation from the SS and bringing it a little closer in electropotential to that of the Brass.

You would do well to use insulated flanges between the two fittings to keep corrosion currents to a minimum.

A common failure mode for gate valves is for the screw shaft to corrode and fail, thus allowing the shutter to be free to float in its retainer. If the valve is mounted with the handle uppermost, then the valve will fail closed as the shutter will fall to the lowest point. If failure open is preferred, then mount the valve with the handle at the bottom.

The use of two stainless steel components in the same grade, ie:- 316 would need to be checked that no contact between the 2 items occurs or the components will 'WELD' together. Stainless steel components that can rub, by their very nature, should be of differing grades to prevent this happening. I have used 316 and 316L pipelines but the valves were always Brass or the valve was fitted with a UHMW polymer coating due to the friction welding that happens with low melting points of the chromium. A70 Nuts and Bolts are another where the nut can weld to the threaded shank of the bolt just using your fingers to spin the nut up. I have lost track of the number of times in electrical cabinets using 316 cabinets and partitions held together with A70 nuts & bolts & washers where I have broken the bolt trying to remove the nuts due to the welding effects and the bolts were never torqued up. The washers also tend to affix to the plate surfaces if they turn during application of tensioning.

Check with your supplier and get a warranted document stating the welding effect will not happen.

"I have lost track of the number of times in electrical cabinets using 316 cabinets and partitions held together with A70 nuts & bolts & washers where I have broken the bolt trying to remove the nuts due to the welding effects and the bolts were never torqued up"

Just a tip - next time, coat your mating surfaces and threads with Never-seez mariners choice or one of the high temp SS grades, and galling will be a thing of the past.

Use of Never Seize paste is prohibited in electrical cabinets in OZ.

That's interesting news. I have been involved in the designing and building of these cabinets in Australia for many years, always use never-seez, never heard of that prohibition, and have never been picked up on its use by any inspectors or site engineers. Obviously been lucky, what's the reason given for the ban?

Perhaps you can point me to the relevant Australian standard or wiring rule.

Over exuberant use of excess amounts of never seize has been found to drip onto contact on electronic boards in underground starter and MCCs tranformers etc and cause flashover and short circuits that have enlivened the surrounding metal which had given electrical personnel shocks and it became a mines notification to not use never seize around locations where the excess could cause an issue.

Thank you for your explanation.

I take it then that its use is not actually prohibited by electrical authorities in Australia, but just by some mining companies on their own projects.

That is likely why I was unable to find any reference to such prohibition in the Standards and have never been aware of any other usage restrictions as mines work was not one that I was involved in.

I do accept that inattention to cleanup after tightening could cause over-run and drip concerns

In reference to #8, the following is from a post on 8/17/14,

http://cr4.globalspec.com/thread/97203/Seizing-Binding-of-Stainless-Steel#comment1038340

Sorry about the length but it self explanatory:

Cold drawn 304 and 316 are the best for anti-galling (to not seize). Galling is caused by "micro welding" on micro high points on the surfaces of both the male and female threads. As the threads are tightened the breaking high points lose oxides and these points weld themselves together. The solution for this problem is to either reduce the tightening torque, change one of the alloys to another metal, or use a friction reducing material on the threads. For bolts and nuts a reduction in tightening speed or the use of a lubricant is most often used. For piping the male thread is often either covered with a lubricant containing Teflon powders or Teflon pipe dope tape.

http://www.cartech.com/uploadedImages/Tech_Center/Technical_Articles/Articles/Austenitic_Galling(2).png

This chart shows Threshold Galling Stress of various materials. Note that there is a substantial difference between cold drawn and annealed stainless steels.

Mix 304 with 304 or 316 with 304.

http://www.estainlesssteel.com/gallingofstainless.html

At least in the USA either Teflon tape or a pipe dope with Teflon, such as "LOCTITE Thread Sealant With PTFE, #30534" or similar products are used for tapered pipe threads. Straight threads such as those on bolts and unique pipe configurations are normally coated with an appropriate quantity of "Never-seez, Stainless Steel".

http://www.bostik-us.com/files/tdsfiles/NSHiTempStainless.pdf

Not familiar with the Ausie methods, but here in the USA the above is standard practice in the chemical industry, probably the largest users of St Stl in this country. Very seldom have I seen St Stl pipe with other than St Stl valves. In some cases we would use St Stl piping and valves for installations where lesser materials would be sufficient. These were almost always situations where access was difficult therefore being expensive for down time repairs. Steam piping was the most frequent type for Stl Stl instead of cheaper materials.

101 years of good experience with stainless steel has proven it to be a alloy with well know properties since it was discovered in 1913 by Harry Brearley. That's a long time for the US chemical industry to get to know it!

Good Luck, Old Salt

There are plenty of plastic or GRP valves available in threaded or flange configurations that could easily be coupled to SS pipe and will handle up to around 10 bar. No dissimilar metal corrosion problems that way.

Thanks all,

Dear Spade, your point is very important about galvanic corrosion, but one question came to my mind, is this issue also happens on thermocouples where 2 metals in conjunction point?

For fittings, we are planing to remove the PVC pipe and replace SS 316L, but there are existed soleniod valves made of brass! they will be threaded connected to SS pipe, is there any isolation method to aviode dissimilar metals.

It is pretty much impossible to electrically isolate metal fittings which are threaded together, and so galvanic action between the two metals will be inevitable. How much galvanic action is going to take place will depend on the alloys involved and the composition of the electrolyte.

A look at a Galvanic series chart should give you a starting point for what to expect, with the less noble metal (in this case the Brass) being depleted.

The rate of depletion will depend on numerous factors such as, but not limited to, electrolyte composition, external electrical continuity, separation distance of the two metals in the series, etc.

Note that most series charts are based on a sea water electrolyte, and the relative positions of the metals in the series can alter substantially with different electrolytes.

For galvanic action to occur, there needs to be three factors in play.

1. The metals must have different corrosion potentials as per the Galvanic series. The further apart these are in the series, the more corrosion potential exists.

2. A suitable electrolyte that is in contact with both metals. The more electrically conductive the electrolyte, the higher the corrosion risk. Note that in many charts, 316 SS will be indicated at vastly different positions depending on whether it is passive or active. Its passivity will greatly depend on the aggressiveness of the electrolyte, and whether that will keep the passive layer from reforming on the SS. In the case of Brass and SS combinations, the lack of a passive layer actually reduces the corrosion risk to the Brass by bringing the 2 a little closer together in potential without any detriment to the SS.

3. An external electrical connection (return path) for the corrosion currents.

Remove any of the 3 above points and corrosion cannot occur, and the only one of these that you will be able to control is point number 3.

If at all possible, I would suggest fitting flanges to the valves and pipe and using flange isolating kits. Any decent engineering shop could fabricate flanges to suit, obviously of Brass for the solenoids and 316L SS for the pipes. This also makes valve replacement so much simpler.

You also need to be aware that mains voltage solenoid valves may need to be earthed, and if the pipe is in any way in contact with that earth, then you will have re-introduced point 3. If possible, use double insulated valves or better still ELV ones to remove earthing requirements.

An alternative but possibly expensive one (depending on the number of valves that require earthing is to supply them via small isolating transformers thus eliminating the need to earth, but you will need one for each solenoid to avoid second fault dangers.

If having flanges made, make sure the insides are as smooth as possible and that welds are complete to avoid areas in the structure which will allow stagnation, as that can introduce other problems such as crevice and pitting corrosion to which SS can be particularly susceptible.

Forget about all the potential galvanic effects and other potentially expensive and worrisome problems. Switch all the valves, including solenoid valves, to 316 Stainless Steel valves. With the identical metals there are no problems, problems you might not be aware of now but could be expensive in the future. Depending on what size, make and model you may be able to use the same operators as before but mounted on new st stl valve bodies, usually a relatively simple task. Why take the risk?

If what you are saying in #17 about the PVC piping is true, you have been taking serious and very illegal (in the USA) risks with your personnel. PVC in compressed air use is very dangerous! Since air is compressible when the pvc develops a leak the forces don't stop until the pressure is all gone inside it. Combine this with the fact that PVC will break up in shards (sharp and numerous pieces, some large and some small) with the force behind them they are as bad as bullets being fired at your personnel and YOU. People have died from the shattering of PVC in air service. It has the same effect as a Claymore Mine, (fragmentation) used in the Viet Nam war. OSHA (Occupational Health and Safety Administration, a federal group) has banned its use for compressed air.

https://www.osha.gov/dts/hib/hib_data/hib19880520.html

https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=NEWS_RELEASES&p_id=21567

Make life simpler, safer and less expensive now and for the future, go with 316 stainless steel valves and other components!

Good Luck, Old Salt

despite our service is compressed air on the pipe, there is no electricity, but I think potential is generated due to friction between the air and the pipe, specially the flow is 200 l/min.

PVC pipe and fittings, which are illegal to use in the US for compressed air, could create a static electricity within the pipe if the air is completely dry. Not a normal problem because any condensate within the pipe will greatly reduce the possibility for that problem.

If there still is a continuity problem in metal piping due to gaskets or other insulators, use a bonding wire clamped to the piping on both sides of the insulating fittings. Make sure you use bonding clamps that slightly indent the pipes to firmly hold them together and provide a constant connection between the wire and the pipe. Ground the wire at least at both ends. More often is even better. If you are still hesitant, use one continuous wire from end to end with the clamps on each section of pipe. Be sure to ground it at each end. Grounding at several places is even better if the run is long. Use solid copper wire if possible. It is the best for the least $.

Good Luck, Old Salt