Potential for corrosion with zinc plated, zinc yellow chromate plated, and mild steel parts?

[kornfeld]

Hi, noob here.

I'm machining a few small parts for usage on the floor inside of a car. So, it isn't a boat or anything, but there is definitely the potential for it to get damp occasionally.

I'm trying to use as many off-the-shelf parts as possible, but the machined component is made out of mild steel. It's basically just a piece of mild steel that's internally threaded to accept a bolt. Then I'm hoping to thread a regular bolt into it.

Here are my questions:

1. If I thread a zinc-plated bolt into that mild steel part, along with a zinc zellow chromate plated washer, how severe is the potential for corrosion between any or all of those components?

2. What can I do to prevent corrosion in the threads? Will loctite help with this?

3. Any other thoughts or suggestions?

Thank you in advance.

 

[pontiac428]

1. If I thread a zinc-plated bolt into that mild steel part, along with a zinc zellow chromate plated washer, how severe is the potential for corrosion between any or all of those components?

Since yellow zinc does not spontaneously corrode the bolt, I would think there's little concern here. Remember, galvanic circuits require a salt bridge, or else they're not galvanic cels. That occurs in engines because the coolant serves as an electrolyte. It's most severe where anodic parts are insulated from the surrounding metal, like metal sleeved rubber bushings in a pipe. When the coated bolts are in contact with the base metal, not much will happen- it's like a side chain circuit there. I know that bridges and the statue of liberty have shown signs after 100 years, but those are collosus constructions that are bound to have strange effects like resonances or electron accumulation.

This comes up a lot. It's like we stop analyzing when we encounter a difference in galvanic potential, instead of seeing if the conditions are even right for galvanic corrosion to occur in the first place. Potential is just potential, it takes ions and electrons to make corrosion, and it takes salt and electrolyte of sufficient strength and density to conduct. Without that, the difference in potential drives only very weak interactions.

 

[Deleted member 69715]

If for some reason you chose not to use any zinc fasteners, get yourself some anti-seize. Coat your threaded parts and assemble. Works a treat.
DO NOT get the anti-seize on your clothing. My wife says the only thing that will get it out is scissors.

 

[kornfeld]

Since yellow zinc does not spontaneously corrode the bolt, I would think there's little concern here. Remember, galvanic circuits require a salt bridge, or else they're not galvanic cels. That occurs in engines because the coolant serves as an electrolyte. It's most severe where anodic parts are insulated from the surrounding metal, like metal sleeved rubber bushings in a pipe. When the coated bolts are in contact with the base metal, not much will happen- it's like a side chain circuit there. I know that bridges and the statue of liberty have shown signs after 100 years, but those are collosus constructions that are bound to have strange effects like resonances or electron accumulation.

This comes up a lot. It's like we stop analyzing when we encounter a difference in galvanic potential, instead of seeing if the conditions are even right for galvanic corrosion to occur in the first place. Potential is just potential, it takes ions and electrons to make corrosion, and it takes salt and electrolyte of sufficient strength and density to conduct. Without that, the difference in potential drives only very weak interactions.

Ok great, thank you for that info. Just to make sure I'm reading you correctly, when you said, "Since yellow zinc does not spontaneously corrode the bolt," you're referring to the zinc not corroding the bolt that it was plated onto, is that right? Meaning, the zinc doesn't ruin the bolt it's attached to, and therefore it's unlikely to corrode the other part as well, unless there is an electrolyte to create the conditions for a galvanic cell?

 

[kornfeld]

Also, just to clarify on this single point--is there any issue mixing parts that are zinc plated with those that are zinc yellow chromate plated?

 

[Nutfarmer]

I would use the mild steel and not worry about it. If you want to protect the outside of the steel part paint with some red damp primer.

 

[Jake M]

Here are my questions:

1. If I thread a zinc-plated bolt into that mild steel part, along with a zinc zellow chromate plated washer, how severe is the potential for corrosion between any or all of those components?

The plating is your friend on the parts that are plated. All these parts are low/mid carbon steel, they'll be fine with each other, no galvanic shenanigans are gonna happen there. The plating WILL BE DAMAGED when you tighten things up anyhow, that's just how platings are. . It's mostly just to keep the visible part good. There are other reasons, but they get sciency and far exceed what I speculate to be the scope of this project.....

2. What can I do to prevent corrosion in the threads? Will loctite help with this?

Loctite will indeed help with corrosion. It's not perfect, in the worst case, but within reason (and probably for your project) it'll work just fine. Wicking types are good for that application. 290 to start your search with, and you can pay as much as you want from there.... Or the standard flavors can work too, just make sure it's worked back and forth to cover everywhere.

Another option is fluid film and/or one of the many "reduced scent" varieties of that. Wool Wax is a good one. A quick light spritz and wipe it off as best as you can with a rag, like you were trying to clean those parts. No cleaners or solvents, just a rag. (That leaves a thin enough residue that even the original fluid film "stank" is all but gone, and any odor to the most sensitive of noses is gone is gone in days. Out of the weather, that stuff will stay ten years. But not if you plan to paint any of it.

3. Any other thoughts or suggestions?

Put the whole thing together and paint it as an assembly.

Thank you in advance.

 

[kornfeld]

Fantastic, thank you to each of you for the input. And I'll presume from the lack of comments about it, that combining regular zinc-plating with yellow-chromate zinc plating is not a concern. Thanks all!

 

[Jake M]

And I'll presume from the lack of comments about it, that combining regular zinc-plating with yellow-chromate zinc plating is not a concern. Thanks all!

Absolutely not a problem. They'll coexist just nicely. The car you're putting this in more than likely has five hundred or a thousand points where different zinc based platings interface, various platings to plain unplaced mild steel holes, etc.

 

[pontiac428]

Ok great, thank you for that info. Just to make sure I'm reading you correctly, when you said, "Since yellow zinc does not spontaneously corrode the bolt," you're referring to the zinc not corroding the bolt that it was plated onto, is that right? Meaning, the zinc doesn't ruin the bolt it's attached to, and therefore it's unlikely to corrode the other part as well, unless there is an electrolyte to create the conditions for a galvanic cell?

Yes, exactly. Metal to metal contact conducts electrons freely, but does NOT create a galvanic cell. I'm not minimizing the need to preserve against such corrosion at all- it is real when the system supports it. Every car on the road uses yellow or gray zinc throughout the undercarriage, and even on salted roads they hold up. In a galvanic system, you can insert volt meter probes and see the differences in potential anywhere in the circuit. If I am on dry land and touch a zinc plated bolt threaded through bare steel with one probe and the plate with the other, I won't see a difference in potential, because it is not a galvanic cell despite the presence of dissimilar metals. People are often misguided on this, they learn something and think it applies everywhere, all the time.

Also, just to clarify on this single point--is there any issue mixing parts that are zinc plated with those that are zinc yellow chromate plated?

The two coatings exhibit different oxidation states of zinc, but they are both stable. Oxidation state does not affect galvanic potential right out, you need to work the half-reactions of each chemical change that results first. In practice, it's unnecessary. If you're building a boat, use stainless. If you're building high tension towers, hire a special engineer. For machinery and vehicles, you won't live long enough to see any effect at all. When zinc coatings start to get powdery, it's because the zinc (terminal electron receptor) is reduced as a sink for oxidation, and that is how it is supposed to work. It's not an indication of something wrong with compatibility.