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Prevention of Galvanic corrosion Print E-mail


One type of corrosion that is often overlooked in the moldmaking industry is galvanic corrosion. Sometimes it is identified wrongly as electrolysis. Galvanic corrosion refers to the damage induced when two dissimilar metals are coupled in a corrosive electrolyte. When this occurs, the less noble (less able to resist this type of corrosion) of the metals in the reaction becomes the anode (positive) and corrodes more quickly than it would by itself, whereas the more noble metal becomes the cathode (negative) and corrodes more slowly than it would alone.

An example of this corrosion phenomenon is increased rate of corrosion of steel in seawater when in contact with copper alloys. Galvanic attack can be uniform in nature or localized at the junction between the alloys depending on service conditions. Galvanic corrosion can be particularly severe under conditions where protective corrosion films do not form or where they are removed by conditions of erosion corrosion.


Prevention or Remedial Action
  • Selection of alloys which are similar in electrochemical behavior and/or alloy content.
  • Area ratio of more actively corroding material (anode) should be large relative to the more inert material (cathode).
  • Use coatings to limit cathode area.
  • Insulate dissimilar metals.
  • Use of effective inhibitor.
The first step that to understand galvanic corrosion. This will help to avoid making false assumptions and accusations.

One of the most common methods of avoiding galvanic corrosion is to use a sacrificial anode. Here's the logic: suppose that you are using a nickel solution to plate an aluminum part. Now, if there were any imperfections in that aluminum part, such as pitting, there is a possibility that during the plating process some of the solution or a water molecule could get trapped in that deformity in the part. That water molecule ultimately could become the electrolytic component needed to complete a galvanic cell. The aluminum is less noble than the nickel and so in the plating process used to prevent corrosion and to prolong the life of your part, one could actually be promoting corrosion, and lessening the life of your tool. The solution is to use a sacrificial anode.

To ensure that galvanic corrosion will not occur, even if there is an imperfection in the aluminum, a metal should be placed in the solution that is less noble than both nickel and aluminum to sacrifice electrons to the aluminum and endure the corrosive reaction created in the process. This isn't applicable in all situations, but when available, this process has proven itself most beneficial.

One of the more common ways that galvanic corrosion occurs is after the mold has been taken out of the tank. Consider for example, placing an aluminum part in a nickel-plating solution. The aluminum part again has some pitting that hasn't been treated by a resurfacing process. Aluminum and nickel, two obviously dissimilar metals, have been bonded with each other. However, in the pit less nickel has been deposited than on the rest of the piece, an aluminum pit being exposed to nickel - now it is a nasty electrolyte to complete the process.

This electrolyte in abundance floating freely in the air in the form of oxygen. The condensation in the air coupled with the two dissimilar metals completes the equation needed to create a galvanic cell, and thus the process has started. With oxygen introduced to the mix, the corrosion is able to spread much further and faster than it would have had it been created in the previous example.

It is possible to protect from this reaction to the plating process in one of two ways:
Closely inspect the mold for any pitting or surface imperfections and treat them accordingly before proceeding with your plating.
Make sure that there is at least 0.0005 in. of nickel is being applied to ensure complete .

Prevention of galvanic corrosion by design


Requirements for Galvanic Corrosion

In order for galvanic corrosion to occur, three elements are required.
  • Dissimilar metals (electrochemically)
  • There must be an electrically conductive path between the two metals.
  • There must be a conductive path for the metal ions to move from the more anodic metal to the more catholic metal.
If any one of these three conditions does not exist, galvanic corrosion will not occur. Galvanic corrosion can be minimized in design. For example, the direct contact between the two metals is prevented (plastic washer, paint film etc.) there cannot be galvanic corrosion.

The following practical rules invaluable in this respect
  • Select combinations of metals which will be in electrical contact from groups as close together as possible in the galvanic series.
  • Electrically insulate from each other metals from different groups, wherever practical. If complete insulation cannot be achieved, paint or plastic coating at joints will help.
  • If you must use dissimilar materials well apart in the series, avoid joining them by threaded connections as the threads will probably deteriorate excessively. Brazed or thermal joints are preferred, using a brazing alloy more noble than at least one of the metals to be joined.
  • Avoid making combinations where the area of the less noble, anodic metal is relatively small compared with the area of the more noble metal.
  • Apply coatings with judgment. Example: Do not paint the less noble metal without also painting the more noble; otherwise, greatly accelerated attack may be concentrated at imperfections in coatings on the less noble metal. Keep such coatings in good repair.
  • Consider use of cathodic protection.
 
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