What Should Be the Control in a Corrosion Experiment?

[rossverg]

Hi everyone,

I need to do an experiment at home that tests different environmetnal conditions that facilitate corrosion of stainless steel and mild steel screws.
I have 20 screws (10 stainless and 10 mild steel screws). Each screw will be in a separate container. One pair of srews will have the same environmental conditions so I can compare corrosion of the stainless and mild steel screws.

My conditions are:
*Salt water
*Vinegar
*Screw with copper wire attached to it
*Plain distilled water.

I need the remaining pair to be my cotnrol from which I cna make comparisons. The problem is, I don't know what my control would be... Would it be just the screws in an open air container exposed to normal conditions (air with mositre) or would it be the screws in an air tight container with silica gel or would it be a screw in normal plain water?

Some mroe questions:
* Should I half-submerge my screws in their respective solutions or fully submerge them? (I was planning to half submerge them so they could be expsed to air but I don't know if this would make any difference than if they were fully submerged).
* For the copper wire which I want to act as a cathodic site, would it be better to attach the screws to a plate of copper that wrapping a thin wire around it? Would the larger surface srea make any difference?


Thanks in advance

 

[Astronuc]

Well, one can weigh each screw and compare the mass of each screw as a function of time.

A control set would be as is - so keep 2 screws in a dry location, i.e. no corrosion.

Half-submerging the screws reduces the surface area in the electrolyte, so that would have an effect on the corrosion rate.

For the copper wire which I want to act as a cathodic site, would it be better to attach the screws to a plate of copper that wrapping a thin wire around it? Would the larger surface srea make any difference?

Surface area will affect the total corrosion current. Also, read about 'large cathode - small anode' vs 'small cathode - large anode'. Think about which of steel and copper is the anode, and which is the cathode.

 

[rossverg]

A control set would be as is - so keep 2 screws in a dry location, i.e. no corrosion.

Thanks for your help Astronuc.

So my suggestion that I keep a pair of screws in separate air tight containers with a silica gel sachet would be best?

 

[Astronuc]

Thanks for your help Astronuc.

You're welcome.

So my suggestion that I keep a pair of screws in separate air tight containers with a silica gel sachet would be best?

That would work, although I don't think silica gel is necessary - the corrosion rate should be essentially zero.

Also - it is best not to handle the screws with your hands - since they could be contaminated by sweat or oil from the skin. Consider some type of glove or tweezers for handling - make sure hands are clean and minimize contact.

 

[rossverg]

You're welcome.

That would work, although I don't think silica gel is necessary - the corrosion rate should be essentially zero.

Also - it is best not to handle the screws with your hands - since they could be contaminated by sweat or oil from the skin. Consider some type of glove or tweezers for handling - make sure hands are clean and minimize contact.

I didn't think of that. I think I will use tweezers/gloves when inspecting the screws.

Half-submerging the screws reduces the surface area in the electrolyte, so that would have an effect on the corrosion rate.

If I do fully submerge the screws in water, will they still corrode? I'm planning to conduct this experiment over a week but I'm not sure if the water would have enough dissolved oxygen to last (I'll probably only use about 50mL of distilled water).


Just one more point... I know a control is the sample that is exposed to normal environmental condiditions so the other samples (whcih are exposed to other conditions) can be compared to. But if I use the screws in the airtight container as my control, wouldn't it be incorrect becasue I am not taking into account the water/mositure each other sample is expsoed to (like the diluted acide ie Vinegar). Would that still provide an accurate baseline from which I can determine the effect of an acid, lower reactive metal etc on corrosion? That is the point I don't really understand.