ACME corrosion cell on a single piece of metal

Source - https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/20%3A_Electrochemistry/20.8%3A_Corrosion

I took a three-day, corrosion-focused ASM workshop at the University of Akron a few years back. It wasn't necessarily a part of the ASM summer camp world tour, but it was a certainly adjacent to the regular tour. ASM master teachers were teaching it - Andy and Debbie, honestly - and a solid handful of the attendees were ASM master teachers helping out and learning along the way.

That workshop was - I think - the first time that I ever heard the concept of ACME as it relates to corrosion.

By way of introduction, ACME is an acronym for Anode Cathode Metal Electrolyte. In order for corrosion (or oxidation and reduction) to happen, you must have...

• an anode (a more reactive metal)
• a cathode (a less reactive metal)
• a metal (sometimes called a metallic path connecting them)
• an electrolyte (a source of ions that keep the charge of the cell balances)

For example...

Source - https://www.marineinsight.com/tech/understanding-sacrificial-anodes-on-ships/

In the cell there...

• anode - zinc, more reactive than copper, loses electrons, turns from neutral zinc into zinc ions which drift into the solution around the zinc electrode
• cathode - copper, less reactive than zinc, gains electrons, gains mass as copper ions from the solution become neutral copper atoms
• metal - the wire between the zinc and copper electrodes, allows electrons to move from anode to cathode, can involve a thing (light bulb, radio, cell phone) that needs that flow of electrons to opperate
• electrolyte - the solutions around the electrodes and the porous disk that lets the ions move to keep the overall charge on each side of the disk to stay neutral, without it charge would build up and electrons would stop flowing

If this cell is set up, corrosion is going to happen, and the two electrodes don't have to be separated. They can be dissimilar metals abutting each other.

Source - https://pomametals.com/how-to-prevent-galvanic-corrosion/

Here we see two metals joined together (like a copper and a lead pipe connecting) with an electrolyte solution flowing through them. Bad things will happen to the more active (less noble) metal.

If we can break any connection in that cell, corrosion will stop (or at least be drastically arrested).

Source - https://pomametals.com/how-to-prevent-galvanic-corrosion/

After a few years of teaching corrosion in Princeton's material science course and the ASM summer camps and in AP chemistry, I think I'm finally getting to understand the ACME cell.

And I find myself thinking back to what one of the Akron professors said at that workshop when we asked how much of this they wanted us to teach to our students. He said that if students could understand that a single piece of metal could be both anode AND cathode, he would be happy. At the time, I didn't think much about it, but I've come to realize that is a big ask, especially to identify the ACME cell on a single piece of metal.

Thank heaven for that diagram up top (and that I'll repeat here)...

Source - I already told you up top, but since you asked so nicely... https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/20%3A_Electrochemistry/20.8%3A_Corrosion

The ACME cell is still present these on a single piece of iron...

• anode - the bit of iron on the left, for some reason - a crystal defect, a difference in concentration in the electrolyte solution - that site is slightly more likely to release electrons
• cathode - a different part of the iron piece, the part on the right in this diagram
• metal - the two areas of the iron are connected because they're the same piece
• electrolyte - the metal has to be wet with some ions present (keep the metal totally dry, and you prevent corrosion)

At the anode, the iron becomes iron +2...at the cathode, O₂ becomes water (it helps if the solution is slightly acidic)...in between, we have iron ions and oxygen atoms, so we get iron (III) oxide...rust.

See, clear as day, huh?