Dissimilar Metals

[George Geist]

Not sure if this has been discussed around here or not. If so could somebody point me in the direction of where?

If not, It's pretty common knowledge that aluminum and steel don't get along well together. This is why on trucks for example all steel and aluminum is separated by rubber gaskets and such. Question I have for you guys is how well does brass get along with steel? I have a project in mind that I'm contemplating adding brass to but wonder how that metal will be with steel. Anybody know?

George

[Daswulf]

Brazing steel has been around for a while. My first main experiences with it is on automobiles. Brazing sheet metal seams together and such.  I have personally not seen bimetal corrosion from brass and steel like with aluminum and steel. 

 

[Kozzy]

Ignoring whether the metals join together well and easily, it's all about the galvanic series and medium in which the metals are coupled.  On the chart, the metal that is more anodic will tend to be the one which sacrifices to protect the more cathodic---and the potential (voltage) difference comes into play in that the further apart the metals are, the more reactive they tend to be when playing together.

You can see why they put sacrificial zincs on steel ships for example.

It gets a little more complicated also because surface area can come into play as well as shape but that is only a big deal in more specialized areas.  As an example of that, if you suspend a steel part in a stainless steel bucket of salt water, the steel will go south really fast---because the area of stainless around it is so great.  If you suspend the same part made from stainless in a steel bucket, the bucket won't rot through as quickly because it's area is so great relative to the stainless.

[rockstar.esq]

Kozzy,  that's interesting but I'm not sure how to read it.  As an electrician, it's a big "no-no" to connect aluminum and copper directly because the galvanic reaction just eats both metals.  We're typically using tin plated hardware to connect the two, or "bi metal" steel which isn't well defined.

I'd love to know more about this stuff.

George,

I have a few cooking utensils that are brass at the food end and steel for the handle.  Going on a decade old and no signs of any problems.  I've also got a Case pocketknife with brass stock separating the steel blades so I'm guessing that brass and steel get along fairly well. 

 

[George Geist]

Thanks guys,

That's precisely the information I'm looking for. I never saw brass trim ever hurt a knife either so is kinda what I was thinking.

George

[Frosty]

Brass and steel get along fine unless it's a corrosive enough environment, sea water isn't corrosive enough. If there's a question marine of silicon bronze should last a few centuries.

Frosty The Lucky.

 

[Kozzy]

21 hours ago, rockstar.esq said:

 As an electrician, it's a big "no-no" to connect aluminum and copper directly because the galvanic reaction just eats both metals. 

In that case, you are forcing a reaction by shoving electrons through the junction (very much like electro-plating).  In the presence of oxygen, you end up forming aluminum oxide and some variations on cupric oxides, both of which act as resistors to the current flow---and heat up.  Heat is the real thing that causes the freak-out of these electrical connections.

The main purpose of that jelly that's required on these connections is to exclude the oxygen from the joint.  Without oxygen (in theory), the joint can't form ions which in turn can't react with oxygen and form more resistive by-products.  Also, you aren't losing good metal to those by-products which could turn the wires brittle over time.

If you put a copper plate and an aluminum plate separated in a kettle of electrolyte---saltwater in this chart's example, they'd act as a battery with a voltage as stated in the chart by using the difference in potentials--in this case, you'd measure somewhere between .36 and .62 volts.  

Copper potential = -.28 to -.36 V

Aluminum potential = -.7 to -.9V

Circuit potential = (-.9V) - (-.28V) at the top end = .62 volts. and (-.7V) - (-.36V) at the bottom end = .36V....

The bigger the potential between the two metals, the more that one will corrode over time and exposure.  Since graphite is at the far end of the chart, that's why it was used in many of the old school batteries as one electrode (high graphite carbon bar at the center of the old C cell batteries for example).  Lots of voltage potential relative to the other plate on the galvanic chart.  That standard 1.5V battery you see is simply the galvanic difference between graphite and zinc (only 1.33V with seawater as the electrolyte but a little higher with acid)  

Similar in a lead-acid battery like used in cars.  In that case you have plates (after charging) of lead and lead dioxide which have a voltage potential difference of about 2 volts in an acid electrolyte.  6 of those hooked up on series in the plastic box gives you the 12 volts and change.  The difference is the ability to re-charge the dioxide plate while driving so it doesn't just continually eat away the lead (basically re-plating as you drive or have it hooked to a charger)

 

[the other dave]

Actually brass and iron do not get along under certain conditions.  Using a brass valve in a steam line will cause the brass fitting to become sponge copper over time through a corrosion process called de-zincification, (brass being an alloy of zinc and copper).  And I had a brass fitting start leaking after about 15 years on a buried water line because the person who installed the system used a brass coupling between a buried iron pipe from the well pump and a pvc pipe running to the house.