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Aluminum chloride pitting

  • 1.  Aluminum chloride pitting

    Posted 6 days ago
    Hello everyone!

    I am looking into an interesting failure of an aluminum condenser coil and am looking for any insight you can provide.  Here is what I have been told:

    1. The coil isn't supposed to get wet, but it is understood that it will experience "casual" moisture (yeah, not sure what casual means, sorry).  They are exposed to evaporatively cooled air.
    2. Multiple coils are failing.
    3. The supply water is municipal and contains substantial chlorides (on the order of 200 ppm).
    4. Visual examination revealed both general and pitting corrosion.

    What I am getting into here is corrosion rates v. chloride level in the water.  The failures are occurring due to pitting which allows refrigerant to leak.  I am wondering if anybody has information or a reference regarding how chloride concentration in the bulk supply may affect pit growth... my assumption is that it will have little effect on the microclimate in the pit but I have not found anything clarifying this.

    Thanks for any input you can provide!

    ------------------------------
    Todd Springer
    Senior Consulting Engineer
    Augspurger Komm Engineering
    Phoenix AZ
    ------------------------------


  • 2.  RE: Aluminum chloride pitting

    West Michigan Admin
    Posted 6 days ago
    Good afternoon!

    As you probably already know, Chloride ions and aluminum don't play nice together. In aluminum, pitting corrosion is regularly associated with chloride in the water. The chlorine locally attacks the oxide layer, revealing the aluminum beneath for continued attack. For lack of better terms, the ion sitting locally and attacking is why there are pits instead of general corrosion.

    Pitting corrosion is something anodizers have to protect against by making sure their rinse tank water supplies are "clean" - high chloride levels in a rinse bath can lead to pretty extensive pitting corrosion.  

    I don't know that I have any "charts" on concentration vs. corrosion rate, but I would bet they are out there somewhere. Solutions to the problem are to use deionised water or some other treatment process to reduce/eliminate the chloride content. Anodizers can also add a small amount of oxidizing agent (e.g. nitric acid) to the rinse baths to contain the problem. 

    Are these failures unique in the life of the equipment (e.g. are these replacement coils for previous coils that didn't fail)? - I wouldn't count out a metallurgical / processing issue. Aluminum corrosion can be influenced by level of alloying components. The chemistry window for an alloy can be pretty generous, which is why it is a good idea to specify a narrower range in contracts (e.g. 300349 as opposed to a generic 3003). One cast may be on one end of the window and perform well while the next may be on the opposite side and not perform well, all while being a properly cast "3003" alloy.

    The extrusion process could produce susceptibility to corrosive attack. And coarse particles in the "matrix" (e.g. TiB2 or spinels) from improperly cast billet may also be a cause by giving attack-prone sites. 

    Hope this helps.

    ------------------------------
    David Betz
    Sr. Laboratory Engineer
    Hydro Aluminum Metals, USA
    dbetzasm@gmail.com
    ------------------------------



  • 3.  RE: Aluminum chloride pitting

    Posted 6 days ago
    Mr. Betz,

    Thank-you for the response.  I have been given very little information and won't likely get much more... it's a long story.  As you mentioned, all I know is that it is 3003 Aluminum, and I don't have a more specific alloy designation.  I am pretty sure that the issue doesn't have to do with manufacturing as coils from multiple manufacturers and vintages are failing in the same environment.

    I have found literature addressing the initiation of pits, and material loss v. chloride concentration, but nothing related to pit growth specifically.  At the end of the day, I am going to tell my client that the coils can't be exposed to this environment if they want them to last.  I just thought I'd check here before I finalized anything.

    Thanks again for the insight.


    ------------------------------
    Todd Springer
    Senior Consulting Engineer
    Augspurger Komm Engineering
    Phoenix AZ
    ------------------------------



  • 4.  RE: Aluminum chloride pitting

    Posted 5 days ago
    Consider that the evaporative cooling of the air probably involves spraying water through a passing air stream. The droplets evaporate, cooling the air, but the salt in the water is concentrated many times, so 200 ppm could become 20,000 ppm - equivalent to seawater.
    It is well established that aluminum does not do well in seawater, or in marine atmospheres. 
    An effective mist eliminator, between the water spray and the condenser coils, might mitigate the problem.

    --
    John Grubb