Water Pipe Fittings - Materials and Corrosion

Chromium coated brass and stainless steel (austenitic) are very close in the galvanic series. You should define the type of stainless steel, and be aware that galvanic corrosion is also a function of the area ratio of the two metals in contact and the type of fluid (seawater, fresh water..)

corrosion & rust protection

The stainless steel is 303. thanks

lynlynch is close, a schedule 20 will last longer, and a 309 even more and the best a 316L I hope the chlorine content is not too high. But I expect more of PVC or CPVC.

What kind of water do you want to run through the system? Or around the system. What is the environment?

Drinking water (pH=7.5-8.0) supplied by a municipality run through the pipe. Pipe is above ground and iside a chamber. thanks

Yes. your pipes will fail in 35 years. Plan ahead!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Tip: if it is just a line, you'll be trouble free to switch at the end of your 303 to PVC (cold) or CPVC pipe for 3 to 4 feet and then connect to your chrome piece.

The corrosion resistance of SS 303 is lower than that of SS 304, so you've to be carefull and consider the chloride content. the "safe" limit for a SS 304 is around 200 ppm...

http://www.azom.com/details.asp?ArticleID=964

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corrosion & rust protection

Galvanic cell Is a name applied to corrosion of two different metals in contact with each other and joined by a salt bridge. The water characteristics are important as are the types of metals used and will help to determine how corrosive the water will be on the metals in contact. Generally the chrome is plated to brass but is not in direct contact with the water. Therefore only the brass and stainless steel are in contact. Strider6 has given you links to different metals and they show the nobility of the metals. We are usually only concerned if the metals are far removed on the scales indicated and the electrical potential is at its greatest. Brass is an alloy of copper and zinc and is usually already de-zinced if used in water. Look for the symbols CR or ZRC stamped on the brass as these symbols will indicate corrosion resistant.

Of equal or more importance are the characteristics of the water or electrolyte the metals are in contact. The municipality can provide more information than just the pH. You should get enough information to allow you to calculate the water's capability to corrode fittings and is called Langlier Index. This scale is derived from Alkalinity (as CaCO3), Hardness (as CaCO3), Temperature °C, and pH measured. You will need to calculate this index at the different temperatures (usually hottest, coldest, and average expected) and combine it with the variable pH. With the calculations tabled, you should be able to have an indication of the level of corrosion to be expected at various times of the year or water quality. An index less than 0 will be corrosive and the lower or more negative, the more corrosive the water. Generally, cold water and low pH is more corrosive (low index) than hot and high pH. I have commonly used an index of -0.5 to +0.5 as acceptable for most use, with -1.0 indices or lower getting very corrosive. How far you can deviate the index from will ultimately depend on final use.

Corrosion will also be affected by oxygen and chlorine, but if you have selected good materials this corrosion should be minimized and the fittings and stainless will last a long time.

Some nuisance bacteria of iron and sulphur can speed up corrosion rates as can the presence of hydrogen sulphide.

Treatment for corrosive potential is often treated by the addition of polyphosphate or PP. It is sometimes added by the municipality so you may want to check with them. PP provides a protective monomer coating to plumbing but must be constantly maintained and fed into the system at 1-2 ppm.

Hope this helps.

Langlier's Index

Langlier calculator

Use WRC-approved materials.

Simplesζ.