52981

Why bronze needs cosmetic chrome/nickel plating? [Missouri] 

August 26, 2009

My company purchases a porous bronze part that once is assembled, in the field will be exposed to moisture and water (sometimes salt water) 24/7. The plating is part of specs on the drawing from early 1960s. I have asked the bronze mfg company for the reason(s) and he refers me to the specs on my own drawings. I have exhausted all avenues to find an answer to the need for plating, where bronze itself is used in marine applications as one of the best corrosion resistant alloys.
Please advise.

Michael Modjallal
customer - St. Louis, MO, USA

It is my understanding that bronze will oxidize if not coated with some thing or polished as required.
You might get by with dipping it in a thin hot wax to fill the pores.
It is your spec and someone in the past thought that the plating was required.
One way to find out is to give several unplated parts to select customers with no liability attached to see how they work out in the real world. If you find out that you can skip the plating step, it will not take long to recover the price of the give aways.
I suspect that some of your customers prefer the look of the plated part.

James Watts
- FL

There are two issues at hand. First, this porous material is used to drain water due to condensation in electrical boxes; dipping it in wax fills the micro gaps and can not drain water. Second issue is that when archeologists discover bronze statues several thousand years old, they are intact and at worst a slight greenish film is seen on some parts of them. So, Bronze is not like steel that is eaten up by oxidation. It is an everlasting alloy, more like aluminum. Aluminum is not plated for outdoor use. The question comes back to "why it was recommended to nickel plate this alloy?".

Thanks for your info,

Mike

Michael Modjallal
- St. Louis, MO, USA

Hi. I think the answer lies in the word "cosmetic" in the original question. Bronze is/was often nickel-chrome plated. For example, all kinds of cleats and stuff for boats have been nickel-chrome plated bronze.

Bronze is much easier to reliably and durably electroplate than aluminum or stainless steel, and the aesthetic durability is far greater than electroplated steel. When the nickel begins wearing off of bronze, it still looks and acts great, whereas when the plating wears off of steel you have a real mess.

Regards,

Ted Mooney, P.E.  finishing.com Brick, New Jersey

FYI, bronze does corrode when exposed to corrosive environments. Hard or salt water can corrode bronze pretty quickly. I do sculpture restoration and see these effects on bronze all the time. Aluminum, when used outside gets an outdoor rated anodizing. If your bronze is going to be exposed to salt water, you should make sure you having your bronze cast in Naval Bronze. As for plating, I think that is just a cosmetic choice, although it would help protect the bronze, but it too will also start to break down in a few years. Good luck!

Dan Romo
- Oakland, CA

Dan, the product Michael is having trouble with is made of deliberately porous bronze so that water will drain through it, as explained in his second post. Using a sound material of any type would completely block (no pun intended) that property.

Michael, there are powder metallurgy techniques (essentially where powdered metal is compressed to a shape and then sintered at a temperature below its melting point) which could give you controlled porosity in many different alloys. Cost will be a big factor in considering that route, though.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia