1
$\begingroup$

I have an aluminum hinge that I want to use with a stainless steel hinge pin. The problem is that I cannot use the steel pin directly because there would be galvanic action between the stainless steel and aluminum which will result in corrosion. Therefore I need to put some kind of bearing liner between them. Note that the hinge will experience strong forces, so using something soft like nylon will not work.

My preference would be for a bronze bushing, but I do not know if there is galvanic action between bronze and stainless steel, or bronze and aluminum.

What material can I use? Is there a simple explanation of how I can predict whether a metallic alloy will react galvanically with a different alloy? I have Mathematica, so if there is a formula I can write in Mathematica, that would be just as good.

-------------------------------- Sacrificial Anode?

One method I have heard of to protect from galvanic corrosion is a "sacrificial anode". If I attach a bar of zinc to the back of the aluminum hinge will that somehow protect it, and allow me to use a stainless steel hinge pin?

$\endgroup$
  • $\begingroup$ Pretty much any two different metals would make a galvanic pair. If anything, bronze bushing in your case is worse than no bushing at all. Then again, all this does not matter unless the hinge is constantly soaked in water. Is that so? $\endgroup$ – Ivan Neretin Jan 17 '17 at 17:58
  • $\begingroup$ @IvanNeretin No, but in real life, galvanic corrosion will occur without any visible electrolyte. For example, if you screw a stainless steel screw into an aluminum plate, after a few years there will be visible corrosion on the aluminum. $\endgroup$ – Shaka Boom Jan 17 '17 at 18:19
  • $\begingroup$ In a sense the hinge will effectively be "soaked" in water at the level of a few to dozens of molecular layers of water, depending upon the relative humidity at the hinge, which is plenty to provide a medium for an electrolyte layer promoting corrosion over months and years. $\endgroup$ – airhuff Jan 17 '17 at 18:50
1
$\begingroup$

If you must use an aluminum hinge with a steel pin, anodize the aluminum. Anodizing builds up a coat of very hard, non-conductive $\ce{Al2O3}$, sapphire.

There are commercial metal fishers that will perform this, if you do not want to do so. There is a bit of an art in the process of creating a uniform coat of the required thickness: too thick and it cracks off; too thin and it wears off.

$\endgroup$
0
$\begingroup$

The sacrificial anode is certainly a possible and common approach, but I will propose another approach using organic polymers (not “soft” , not Nylon).

According to the Engineers Edge website:

There are three conditions that must exist for galvanic corrosion to occur. First there must be two electrochemically dissimilar metals present. Second, there must be an electrically conductive path between the two metals. And third, there must be a conductive path for the metal ions to move from the more anodic metal to the more cathodic metal. If any one of these three conditions does not exist, galvanic corrosion will not occur.

Note that if you are not convinced of my proposed organic polymer approach that the link above includes a table of anodic indices for several metals and alloys.

Organic based corrosion inhibitors can strongly chemisorb to and effectively passivate many metal surfaces, including stainless steel and, I assume, brass. You only need it on one of the two. This chemisorbed layer prevents the formation of a water/electrolyte layer at the metal's surfaces. Additionally, this layer inhibits the transfer of metal ions. So this mitigates the second two out of the three criteria given above for galvanic corrosion to occur.

Pollyurea-based elastomers are commercially available and should be an excellent candidate in your case. This would be simple to implement and you have a good choice of commercially available products (here and here just for example) of varying physical characteristics. These types of elasomers are used in situations that are probably far more physically rigorous than your hinge would experience: sealing metallic joints in the hulls of maritime cargo vessels for example.

According to this Wikipedia page:

Some polyureas reach strengths of 6000psi (40MPa) tensile and over 500% elongation making it a tough coating.

Also sited in the same Wikipedia reference is an interesting line of research being taken to even further pollyurea-based elastomers’ already desirable characteristics:

In 2014 a polyurea elastomer-based material was shown to be self-healing, melding together after being cut in half, without the addition of catalysts or other chemicals. The material also includes inexpensive commercially available compounds. The elastomer molecules were tweaked, making the bonds between them longer. The resulting molecules are easier to pull apart from one another and better able to rebond at room temperature with almost the same strength.

The rebonding can be repeated. Stretchy, self-healing paints and other coatings recently took a step closer to common use, thanks to research being conducted at the University of Illinois. Scientists there have used "off-the-shelf" components to create a polymer that melds back together after being cut in half, without the addition of catalysts or other chemicals.

Literature references are given in the previous link.

The options I’ve discussed or referenced above focus on the choice of metals used for each component of the hinge or as a sacrificial anode (see table in Engineers Edge link) and a polyurea-based coating. I propose the polymer coating approach for the arguments made above including ready availability, affordability, likely effectiveness and relative simplicity.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.