In this case the plating was indeed the cause of the corrosion. Depending on what end of the galvanic series you are on the plating can help or hurt. In this case, the plating was nickel and the base material iron. Since the nickel is more noble than the iron, the iron sacrificed itself to save the nickel. Normally, if you are going to plate something for corrosion protection you will apply a less noble element. There are very few materials that are less noble than iron. Zinc is one of them and is why it is plated onto steel and iron. It will sacrifice itself to save the iron.
One could argue that even with a more noble plating material than the base material you still need a hole to start the process off. That is true but what coating is perfect? Particularly with a casting such as a typical butterfly disc. It is very rough and porous. If you look at a cross section of a plated component in a high-powered microscope you will see the voids in the coating. It is virtually impossible to obtain a hole-less coating. Thus, such an assembly is designed to fail. And they did in wholesale fashion. John Hoffman P.E. | Fire Protection Engineer | Facility Engineering Services, KCP, LLC - Burns & McDonnell Engineering | National Nuclear Security Administration's Kansas City Plant | Operated by Honeywell FM&T | 2000 E. 95th St | Kansas City, MO 64131 | ph 816-997-7213 | [email protected] From: å... .... <[email protected]> To: [email protected] Date: 01/29/2011 05:49 AM Subject: Butterfly vs. Gate valves: corrosion Sent by: [email protected] consider the argument: the BFV failed because the disk was plated. if the sole source of corrosion was simply the plating, then pipe plated with zinc would self-corrode as well, in which case, it is a sham and a shameful marketing scheme... but plating in and of itself is not the cause. plating is the potential, or if you will... plating is the stored energy waiting to be turned into what a battery working off chemical electroPOTENTIALS from the different metals. (think of a ball sitting on a hill capable of rolling down, but won't until it is pushed. Eddy may go; but plating wont -- at least until the base metal shows). something has to break the plating, in order to complete the circuit and set up the galvanic cell. if the plating is thin, the integrity can then be easily compromised. If the plating is thick, then corrosion defense sticks it out a bit longer (but no longer than the weakest link in the chain). Few ("never" say all) things evade corrosion. Stainless corrodes, even moly-alloy stainless corrodes. galvanized pipe: not accepted in Mist Systems by FM for the very reason that one can not guarantee the plating has integrity across the entirety of a twenty-one foot stick. which begs the question, why are we specifying zinc plated pipe and not home-rule, old-school Sch 40 black-steel? Scot Deal Excelsior Fire Protection _______________________________________________ Sprinklerforum mailing list [email protected] http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum For Technical Assistance, send an email to: [email protected] To Unsubscribe, send an email to:[email protected] (Put the word unsubscribe in the subject field) _______________________________________________ Sprinklerforum mailing list [email protected] http://fireball.firesprinkler.org/mailman/listinfo/sprinklerforum For Technical Assistance, send an email to: [email protected] To Unsubscribe, send an email to:[email protected] (Put the word unsubscribe in the subject field)
