Putting two different metals in contact in the presence of an electrolyte – which in practice means moisture with anything dissolved in it – produces a galvanic cell. Current flows between them and one of the two corrodes faster than it would on its own. That is not an exotic failure mode; it happens in any container holding mixed metal parts in a humid environment, and it is frequently mistaken for a general corrosion problem.
Which one suffers
The less noble of the two – the one further down the galvanic series – corrodes, and the more noble one is protected. Aluminium in contact with steel corrodes; steel in contact with copper corrodes. The effect is strongest when the two are in direct contact and weaker when they are separated, but in a closed humid container even proximity through a moisture film is enough.
Area ratio makes it worse
The rate depends on the ratio of the two areas. A small aluminium part in contact with a large steel surface corrodes quickly, because the corrosion current from a large cathode is concentrated on a small anode. A large aluminium part against a small steel fitting corrodes slowly. Where mixed metals must share a container, keeping the less noble part large relative to the more noble one reduces the damage substantially.
What stops it
Breaking the electrical path, which means separating the metals physically – a divider, a plastic tray, a sleeve. Breaking the electrolyte path, which means keeping it dry, which is the humidity control already discussed. Or coating one of them, which works until the coating is scratched. In a reusable container the first is usually the practical answer and the cheapest.
Why dividers earn their place here
A container with compartments is not only an organisation aid; it is a way of keeping dissimilar metals apart. Where a kit contains both, giving them separate compartments removes the contact and most of the risk. That is a specification decision and it is the reason some kitting containers are divided in ways that look excessive for the parts involved. The design trade-offs are in kitting box compartment design.
Plating and coating interactions
A plated part is not the metal it looks like. Zinc plating on steel is sacrificial and protects the steel even when scratched, which is why it works well. A decorative nickel or chromium layer on steel is the opposite – it is more noble, so at a scratch the exposed steel corrodes preferentially and quickly. Two parts that look similar can behave very differently, and the plating specification matters as much as the base metal.
What to do with an existing mixed kit
Identify the pairs in contact, and separate them. Where separation is impractical, dry them thoroughly and use desiccant sized for the duration. Where a kit has run for years without trouble and then starts corroding, look for a change – a new supplier with a different plating, a new dunnage material that holds moisture, or a route that now includes a longer closed period.
Recording which parts are affected
Corrosion from this mechanism is localised at the contact points, which is diagnostic: general corrosion means humidity, localised corrosion at a contact means galvanic. Photographing the pattern before cleaning is worth doing, because once the rust is wiped off the evidence is gone. The general humidity question is covered in why metal parts rust inside a plastic box.
What to ask when specifying
Which metals are in the kit, whether they touch, what the plating is, and how long the container stays closed. Four questions at the specification stage prevent a problem that is otherwise discovered by the customer.