‘We condemned and scrapped that solar water heater long ago’, said Dr Bikramaditya Swain, MD of SER’s Hospital. ‘We’ve got a new solar heater on the terrace now.’ I had gone nosing to take some pictures of the old corroded water heater that I had seen during my posting in Garden Reach. It had some interesting corrosion area and I was crestfallen.
Never the less the good doctor gave me a guide to show the new solar heater on the terrace of the Hospital. It was the standard design of finned copper tubes built into a glass fronted box. There were four panels in parallel connected by galvanized pipes from the mains and to an insulated hot water tank. The pipes were connected by flanged joints to the flanged copper headers of the solar collectors.
The copper flange of the header was bolted to the flange of the GI pipe with carbon steel bolts with a rubber gasket in between. Their nuts on the copper side were quite corroded and the gasket hardened and cracked. It was perishing from prolonged exposure to sunlight.
The next view shows the same flange from the other side. Note that the heads of the bolts are in very good condition.
This is classic galvanic corrosion! Where steel met steel there was no problem, but when copper met steel there’s profound corrosion. This is as expected, as can be seen below from the Galvanic Series in seawater at normal temperatures.
The Series is best used for general guidance only. A change in the environment e.g. changes in dissolved chemicals, temperature, and oxygen levels and even velocity of water can modify the ranking. The reason for this is commonly modification of surface film (oxides etc) on one of the metals.
There are several things that could be done to reduce rusting of the fasteners. For one, the bolt head could be placed on the side of the copper flange with the nut on the G.I. flange. That would delay corrosion around the nut, which makes it difficult to open. Another thing that could be done is to give several coats of paint on the copper flange. With better insulation electrical charge will not flow slowing down corrosion. Insulating the bolt shank from the flange by using non metallic washers and sleeves is also an accepted solution.
The problem does not really end with the nut and bolt. The attached piping is galvanized and the zinc coat is sacrificed to protect the steel pipe. Now zinc is anodic to steel at normal temperatures. At over 60oCentigrade, zinc forms a hard oxide which makes zinc cathodic to steel. As solar heaters of this design commonly heat water between 50 and 80o C, the galvanized layer will actually aggravate the corrosion of the steel pipe!
Cathodic reduction of oxygen into hydroxyl ions occurs more easily on a copper surface. In the presence of dissolved carbon dioxide, traces of copper bicarbonate are taken up in the water. These are then deposited as metallic copper (microscopic level) on the steel surface of the galvanized hot water pipe. The zinc and iron now form galvanic cells with the copper deposits, increasing the corrosion at points well beyond the steel copper junction. In Europe where hot water systems used to have galvanized steel tanks in conjunction with copper pipes, systemic failures because of corrosion were common. We may have to learn this lesson ourselves if we do not take a lesson from the European experience.
Ideally we should be using either good quality CPVC/ X-linked PE pipes or copper piping to connect a solar heater. All these are available on the Indian market. The final storage tank should also be of CPVC/X-linked PE but there seems to be no manufacturers. The alternative is to use a stainless steel tank.
Without these changes we will continue to see what I have experienced in many places on the old ER & SER. Solar heaters are installed with fanfare on rest houses, hospitals and other public buildings. Only a few years later they lie neglected and irreparable, from rust, dust and decay.