Hydrogen sulfide attack and corrosion of copper

A. My first job out of college was supervising an analytical laboratory for a major petrochemical manufacturer in Northwest Indiana. I had a variety of Gas Chromatographs in the lab, and one particular unit was dedicated to trace analysis of hydrocarbons, alcohols and ketones in waste water. This water was pulled from Lake Michigan and returned to the lake after circulation through process heat exchangers cooling our cracking and distillation equipment.

After about six month's service, the detector signal began to drift uncontrollably. The service technician found severe corrosion of the "Attenuator switch," a decade box type rotary switch which multiplied signal strength to the chart recorder up to 10 to the power of six times in factors of 10.

Since the lab was smack between the second largest oil refinery in the world and two huge steel mills, we concluded the sulfur ⇦this on eBay or Amazon [affil links] in the air was the problem. We tried spray tarnish removers followed by inhibitors to no avail. The instrument company sent the switch back to their headquarters in Connecticut, where a local plater stripped the silver and plated a heavy layer of nickel topped with a substantial layer of gold. End of problem. In the remaining four and a half years that I worked there, so did the GC, and accurately at that!

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Ed. note:
- Letter 4793, "Protect aluminum from hydrogen sulfide with nickel plating or electroless nickel" offers additional detail on a very similar subject.
- Letter 28003, "Silver Flashing vs. Tin Plating of Switchgear Bus Bar" includes a claim by an electrical designer-installer that tin plating is resistant to hydrogen sulfide.

A. Copper will not corrode in hydrogen sulfide if the atmosphere is dry. Obviously, in a hydro-electric plant this might be difficult. Alternatively, a copper alloy with 20% or more zinc would be significantly more resistant to attack than copper alone.

A. Well, that depends on exactly what tarnish you are referring to. The most probable is oxygen which reacts with copper to form a variety of copper oxides. While not a normal component of air, hydrogen sulfide will really tarnish copper rapidly forming one or more forms of copper sulfide. Hydrogen ions are positive and normally would not react with copper. Acids are a special exception. The other normal gasses (in air) that could be negative ions are not reactive under normal conditions. High heat can change that.

A. Hi James.

24 kt gold plating that is thick enough to have no porosity is resistant to nearly everything, including hydrogen sulfide. If you are saying that the nickel will be plated on the copper and the gold will be plated on the nickel, it sounds perfect, as related by Chuck, above. But if you are saying the gold will be plated first, then the nickel, that might not be as good because the gold and copper may diffuse together depending on the temperature and such, leaving a low carat gold with less resistance.

Meanwhile you don't want the nickel on top because, although galvanic corrosion of the nickel by the gold may not be a realistic concern in some particular circumstances, it's usually a bad idea to put a less noble material on top except when trying for sacrificial protection. Good luck.

Regards,

A. Hello Weerasak. Unfortunately your question is too short and abstract for me to understand. If the part was plated for food service, neither paint nor bare copper is acceptable. The same thing if it was plated for electrical service. But even in the best case it doesn't make sense to me to tin plate something, have it get corroded, polish away the corrosion with metal polish, then paint it.

What is the component, and why was it tin plated, and what is the environment it is in that causes sulfide and chloride corrosion?

Regards,