District Energy - 2008 Second Quarter

Inside
Insights

Opposites Attract:
The color purple and
galvanic corrosion

Steve Tredinnick, PE, Vice President of Energy Services, Syska Hennessy Group

To understand the cause of galvanic corrosion, it is important to understand that all metals have an electromagnetic potential or charge that is measured in volts. Table 1 shows this for common metals. At the point of contact, the more active/anodic or less noble material creates an electric current and gives up electrons to the more passive/cathodic material and will dissolve over time.

The current created by the voltage potential is analogous to a DC circuit or very weak battery. The relative or potential galvanic differences (least noble to most noble or anode to cathode) of one metal to another determine the aggressiveness of the corrosion. Furthermore, the relative area relationship between anode and cathode materials will determine the intensity and the rate of corrosion ( therefore, small anode areas relative to large cathode areas should be avoided).

As can also be seen from table 1, titanium, gold, silver and platinum make great materials for piping since they are at the bottom of the galvanic series, but for

Editor’s Note: “Inside Insights” is a column designed to address ongoing issues of interest to building owners, managers and operating engineers who use district energy services.

Contrary to popular belief (yeah, right!) I was not a ‘babe magnet’ in my younger years, but I always wondered why girls in high school were attracted to the ‘bad boys’ and not us boring geeks. Essentially it is a question of whether opposites attract or whether ‘birds of a feather flock together’ – one of life’s little mysteries. Well, I am no Dr. Phil, and if you are looking for an answer to this question, you are searching in the wrong magazine. However, what I can tell you is that if dissimilar metals are in direct contact with one another, opposites do attract galvanic corrosion. Galvanic corrosion occurs in heating and cooling piping systems in commercial buildings and even your home.

While it is more sensational to think there are little ‘nanobots’ gobbling up piping like mini-Langoliers from a Steven King short story, the corrosion is really caused by galvanic action or a basic electrochemical reaction in the presence of an electrolyte (water) – similar to a DC circuit.

Another nonengineering analogy is basic teenage animal magnetism that needs constant parental supervision to keep the stray electrons (or hormones in this case) completely separated from one another forever and ever or at least until they are 30 years old. Our church youth leaders call this ‘purpling.’ This is not to be confused with the novel The Color Purple by Alice Walker that was adapted for the movie; instead, this is where boys are blue, and girls are pink/red, and there is no mixing of red and blue to make the color purple. In other words, there is always a neutral color between them.

To understand the cause of gal-
vanic corrosion, it is important to
understand that all metals have
an electromagnetic potential or
charge that is measured in volts.

Similarly, this concept of purpling – or opposites attracting – is not a good thing for piping systems. There are steps you can take to mitigate these problems, however.