Chlorination of Water Supply and impact on 'wet' infrastructure
Does chlorine at normal water disinfection doses oxidise metal tanks for water tankers and kiosks?
Two concerns - those of the tanker drivers/kiosk owners regarding the state of the tanks and on water quality regarding residual chlorine levels
The risk of using chlorine I believe relates to some of the compounds it creates when reacting with impurities rather than tanks. The main point however is that this risk has to be balanced against the risk of not disinfecting such as AWD especially in crowded camps. Last time I read a paper about chlorination in UK for instance, the conclusion was the risks of not chlorinating were higher than those associated with chlorinating.
No, residual chlorine in water will not damage metal tanks at disinfection doses. Gaseous chlorine released from water will condense as hydrochloric acid (at low concentrations) and could over the longer term lead to oxidation. This is unlikely to be a problem but metal tanks partially filled with chlorinated water do need to be checked periodically.
Chlorine residuals will reduce over time and, at the temperatures in Haiti, will not last for much longer than 24 hours so to ensure disinfection will need to be monitored. This is particularly important if the distribution network has been damaged and the integrity of the pipes cannot be assured
Regards James Montgomery
Assuming that the tanks are made out of steel, the impact of what is less than swimming pool strength disinfectant levels will be minimal. In any case raw water + air will always have enough oxygen and electrolytic potential to cause corrosion in steel.
This is a non issue - if there is metal oxides present in tanks etc, it will only translate as an increased chlorine demand (i.e. the dif between free & total) as long as they maintain a free Cl2 residual (at agreed levels) into supply it will not impact.
It rather depends on the type of metal being used and how it is protected. Mild steel will corrode quite well in any water chlorinated or not which is why it is generally protected by either coatings or electrolytic protection. The oxygen dissolved in the water or as we are considering dissolved chlorine can take electrons from iron atoms turning them into ions which are then insoluble as the familiar brown ferric oxide (hydroxide after absorbing water). Paint or other coating protect by restricting access to the metal surface by the water.
Other metals can be used to protect such as zinc, either as a coating such as galvanising or separate anodes such as are used on ships but these depend on continuous immersion. In this case the zinc preferentially reacts with the oxidant protecting the steel but is itself protected by its oxide film that is harder than iron oxide and inhibits the reaction. On ships etc the anodes are periodically replaced.
Crucially the point is that low levels of chlorine up to 2 mg/l say do not add much to the corrosive nature of the water compared to the oxygen that is already dissolved and it wouldn’t be very drinkable if there wasn’t any.
Hardness and alkalinity also play a part in the propensity to corrode and there is a derived measure that is calculated from the hardness alkalinity and ph called the Langellier Saturation Index. If this is positive scale will tend to form and no corrosion negative and corrosion may occur. Generally the aim is to be very slightly scale forming.
Aluminium and stainless steel both form protective oxide coatings that self repair. In stainless there is also the confusion of chloride crevice corrosion. Chloride ion is different to chlorine molecules and in high concentrations such as seawater and where fresh water concentrates dissolved solids such as cooling systems can cause corrosion of stainless steels. Typically this occurs in crevices and cracks such as areas where items are joined. This phenomenon is controlled by selecting the correct steel for the duty and eliminating crevices as well as limiting chloride concentration. I believe that it is this confusion between low levels of chlorine and high levels of chloride that leads to this sort of query.
Here is a link to the British stainless steels association website that states that up to 2 mg/l for 304 SS there is no effect.
I would have thought that the tankers should already be stainless steel or be protected from normal corrosion is some way.
Chlorine gas and chlorides are corrosive but I wouldn't expect the normal concentration of chlorine to disinfect water would cause a major problem - after all galvanised pipes are used to distribute drinking water that has been chlorinated and food grade tanks are used for chlorinated water. However it would be good to check the level of free chlorine levels likely to be used and the pH of the water.
These publications may be of interest: http://mtac.isws.illinois.edu/mtacdocs/CorrosionFinRpt/CorrosnFnlRpt00.pdf although it isn't a straightforward read!
If there is a risk then cathodic protection may be an option but I don't know enough about it to be sure: see http://anodesystems.com/Cathodic_Protection/Water_Storage_Tank_Cathodic_Protection.html
This thread is public, all members of KnowledgePoint can read this page.