Chemical question about potential corrosion due to chlorine

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  In our operation areas we are using Chlorine tablets 1.67mg to Chlorinate drinking water on elevated water tanks, made from heavy Iron sheets, we experiencing  the elevated tanks has been damaged, the manager said that: this damage happening due to Chemical interaction between Chlorine and Metallic sheet!, is this right ???? and what is the reasons & solution??? Thank you in advance.

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12 Answers

  2. All very good comments.  I just want to support the comment from Soloman and others who mentioned pH.  Corrosion can be due to biofilms, but also due to an unstable water.  Adding chlorine tends to consume alkalinity, and if your water is already unstable as measured by e.g. LSI, it will become even more corrosive after chlorination.  Using a programme such as STASOFT made freely available by the South African Water Research Commission you can easily assess the stability and determine what to do to make it stable.  

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  3. Some good comments, except for David Russell.  Chlorine should never be used on stainless steel because it destroys the surface layer that gives stainless steel the protection.  Corrosion is rapid.  Salt water, Hydrochloric acid and Chlorine are all highly corrosive to stainless steels.  

    The comment by Steve Russell is correct.  An immediate change that can be made, prior to anything else is to quit using the tablets. We assume you mean Calcium Hypochlorite (about 67%) and not swimming pool chlorine tablets that has other stuff added.   Switch to liquid bleach (sodium hypochloride about 12.5% active) and add the material through a schedule 80 PVC chemical injection quill with a Backflow check valve.  that would add the bleach as water fills the tower for a proper mixing.  (see Neptune, or Koflo Corporation - two of many choices)  PS. We carry these items in stock. 

    Karen Baker's comments are correct.   However, the sulfate reducing bacteria are usually hiding out under a biofilm that can hold many different organisms, including Legionella.  The lack of a consistent chlorine feed give the slime bacteria (these are really the true slime molds - the myxomycytes) that once established are resistant to chlorine. I personally have attacked this stuff with over 3000 (yes, three thousand) ppm; and they still come back in two to three weeks. Chlorine just goes over the top of the film and can not reach the polysaccharide glue that holds them on the surface.  So it is best to keep the biofilms off in the first place.  the good news is that the slime molds themselves are usually not classified as toxic, unlike the e coli which is what the regulations are concerned. But the slime molds stink.  

    Chlorine is effective on e coli but it lacks the ability to get under biofilms; that is where the corrosion is found that Karen Baker was commenting about.   Over the past 30 years both my group, and others, have developed several formulations of alkylamides that I have used effectively to get under biofilms and break them us to allow other biocides, including chlorine to make the kill.  However, when involved with potable water system: those systems must be totally isolated both before, during and until after the final rinse out.  

    The applications on biofilms, and the resultant corrosion concerns, are usually for industrial type of water systems because chlorine additions for potable water systems are usually constant and consistent.

     

    Last week I again found a factory at the end of a city water line with no chlorine in the city water.  That water now as a biofilm problem in eight $5million pieces of equipment.  It probably would not have happened if the city water had maintained an acceptable chlorine residual up to the point of contact with the factory.  I have also found it with residential water systems.  

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  4.  David Russel recommend based on Nickel institute corrosion study is correct and you should coat the tank, but be attend as drinking water is food you have to  look for food compatible certified coatings. 

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  5. chlorine will degrade iron and steel unless it is stainless steel.  You might want to consider draining the tank, and when it is dry and sandblasted clean, coat the interior with a chlorinated rubber paint or an epoxy paint.  Be careful to ventilate the tank during any operations inside it because it is a confined space and can prove lethal to workers in the tank.

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  6. The following reference provides corrosion rates based on free chlorine residuals.  Note that this is for the bulk concentration of chlorine.  I would suspect that the local concentrations underneath a chlorine tablet would be considerably higher than the bulk chlorine concentration in the tank.  https://nickelinstitute.org/media/1673/effectofchlorineoncommonmaterialsinfreshwater_14049_.pdf

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  7. Measure the resudual Cl in water  ...more than 0.5ppm   chroe  react with metal

  8. Ensure that the free residual chlorine levels maintained is optimum.

    LSI is another key factor and a slightly positive LSI of 0.1 is ideal to avoid corossion.

  9. Ensure the tablets are not contacting the iron steel when dissolving - they should be added to a floating basket.  If chlorine has only just be added then there may have been a severe level of biological corrosion already.  Look up sulphate reducing bacteria they will cause holes & pits in the metal which will significantly increase the corrosion of the metal. Chlorine is not that effective on established micro corrosion since these bacteria form tubercles that protect them from the chorine. Water corrosion is related to the LSI (Langlier Saturation Index) this is related to pH, calcium hardness & alkalinity of the water.  In order for the water to stop corroding the metal surfaces this LSI needs to be between -0.5 & 0.5.  If you e-mail me I will send you a copy of the calculation and you can check where your water is situated but you will need to test the pH, temperature, alkalinity and calcium hardness of the water.

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  10. Water pH of 6.9 or less is corrosive toward steel.  Check pH and alkalinity and use sodium bicarbonate to adjust pH > 7.5 which will provide protection.   

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    2. I support Thomas' feedback that the most probable cause of the issue is the pH of the water.  If it is not in the neutral range (i.e. typically, pH 6.5 to 8.5), then you are likely to see metallic corrosion occurring.  Best of luck with resolving the issue. 

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  11. Hi Mohammed,

    In addition to the great information from Jean Arenas, there could be a lack of chlorine or continuous chlorine in the tank in certain areas which could result in Microbiologically Influenced Corrosion (MIC). Potable water sanitizers must be present at all times, properly mixed and not be overdosed....just enough. Sometimes the steel alloy contributes to failure because not all steel is equally resistant to corrosion. Sometimes the way the steel plates are joined can be a factor too.

    In the end, any points of failure should be closely examined to determine the root cause.

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  12. Greetings Mohammed:

    When dosing Cl in elevated tanks the following should be taken into consideration -

    • Cl concentration at three stratified points should be monitored (water column)
    • Tanks should be lined with a suitable epoxy paint to protect the metal
    • Heat will augment the corrosiveness, a mixer may be desired if thermal stratification is present

    Hope this helps.

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