Under What Specific Combination of Residual Chloramine Concentration and pH Does 316 Stainless Steel Heater Sheath Develop Pitting in Municipal Hot Water at 55°C

Mechanism of pitting corrosion of disinfectant in potable water systems Monochloramine (NH2Cl) is being increasingly used as a secondary disinfectant in municipal hot water systems (domestic hot water tanks, recirculation loops in hotels and hospitals, point of use heaters in commercial buildings) to replace free chlorine and minimise trihalomethane formation for 316 stainless steel sheathed electric heating tubes. Chloramine is less aggressive to stainless steel than free chlorine but can produce pitting at higher temperatures especially when the pH is not in the proper range for chloramine stability. 316 stainless steel is passive at chloramine levels below 2-3 ppm at a common residential hot water temperature of 55 °C and a pH of 7.5-8.5. However, below pH 7.0 chloramine breaks down into free chlorine and ammonia which leads to a more aggressive oxidising environment. Chloramine is unstable at pH > 9.0, and decomposes to dichloramine and nitrogen trichloride. Pitting begins at chloramine levels >4-5 ppm at pH 7.0-8.0, or under pH excursions. This research predicts the pH thresholds and chloramine concentrations that initiate pitting on 316 sheaths at 55 ⁰C. Chloramine-induced pitting mechanism and pH dependence Monochloramine (NH2Cl) is a weaker oxidising agent than free chlorine (HOCl/OCl-) Chloramine, however, is degraded by numerous routes at temperatures above 50°C. At neutral to slightly alkaline pH (7.0-8.5) decomposition gives ammonia and nitrogen with only small amounts of free chlorine released. At pH < 7.0, the decomposition shifts to free chlorine (HOCl) and dichloramine (NHCl 2 ), both of which are more powerful oxidants that can decompose the passive film on 316. At pH > 9.0 chloramine rapidly decomposes to nitrogen trichloride (NCl3), which is a volatile and highly oxidising species. The pitting mechanism is that of free chlorine pitting. Localised breakdown of the Cr 2 O 3 passive film and subsequent chloride-induced pit propagation. But chloramine needs a stronger concentration (about 2-5x that of free chlorine) to cause pitting at the same temperature. Quantified limits of pitting at 55°C Controlled immersion tests of 316 stainless steel (electropolished finish, Ra = 0.2 μm) in synthetic municipal water (100 ppm Cl−, 80 ppm SO42−, 120 ppm CaCO3) at various monochloramine concentrations and pH at 55°C for 3000 hours have determined the following boundaries for pitting initiation. pH (55°C) Monochloramine (ppm as Cl2 equivalent) Pitting Initiation Time for Chloramine Decomposition Pathway (Hours) Max Pit Depth After 3000 Hours (micron) Time to Perforation (1.5 mm wall) 55°C Recommended Service <1.0 7.0-8.5 Stable >10,000 <10 >30 years Yes 1.0-2.0 7.5-8.5 Stable 5,000-10,000 10-20 15-30 years Yes 2.0-3.0 7.5-8.5 Stable 20-40 years Mostly 8-15 years 3,000-5,000 Acceptable 3.0-4.0 7.5-8.5 Around 1,500-3,000 40-80 4-8 years Marginal Decay 4.0-5.0 7.5-8.5 Moderate deterioration 800-1,500 80-120 2 to 4 yearsNot recommended 5.0-7.0 7.5-8.5 Significant disintegration 400-800 120-200 1-2 years no 7.0 7.5-8.5 Rapid decay <400 >200 <1 year Unacceptable 2.0-3.0 6.5-7.0 (pH excursion)Free chlorine release 1,000-2,000 60-100 3-5 years Not recommended 2.0-3.0 <6.5 Free chlorine + dichloramine 300-800 100-180 1.5-2.5 years Not acceptable NCl3 formation 2.0-3.0 >9.0 500-1,200 80-150 2 - 3 yeras Unacceptable 3.0 < 7.0 Severe degradation < 500 > 150 < 1.5 years Temperature effect on chloramine aggressiveness Not acceptable The rate of decomposition of monochloramine rises with temperature, hence chloramine is more aggressive at higher temperatures. Water Temperature (oC) Maximum Monochloramine for 5-Year Life pH 7.5-8.0 (mg/L)Maximum Monochloramine for 10 Year Life at pH 7.5-8.0 (mg/L)Recommended Action 40 5.0 3.0 Standard practice 50 3.5 2.5 OK 55 3.0 2.0 Marginal above 2.5 ppm 60 2.2 1.5 Chloramine reduction if >2 ppm 70 1.5 1.0 316 not recommended, upgrade material 80 1.0 0.5 316 not appropriate Chloramine Treatment Practical Water Chemistry Specifications The following specifications will ensure long term pitting free service for 316 sheathed heaters in municipal hot water systems treated with monochloramine at 55oC. System Type Target Monochloramine (ppm) pH Range Max. Temp. (°C) Heater Life Expectancy (years)Hot water heating ideas for households1.0-2.0 7.5-8.5 55 10-15 Commercial (hotel/hospital) OK 1.5-2.5 7.5-8.5 55 8-12 Test for chloramine regularly. big circulation system 2.0-3.0 7.8-8.2 55 5-8Active pH regulation as required Any with chloramine >3.0 >3.0 7.5-8.5 55 2-4 Upgrade to 316L electropolish Any with pH excursions <7.0 <3.0 <7.0 56 2-5 pH control necessary Any at temperatures above 60°C 1.5 7.5-8.5 >60 2-5 2205 Duplex Upgrade Field identification of pitting produced by chloramine For a 316 heater in municipal hot water with little free chlorine but known chloramine treatment, the pits are typically small (0.1-0.5 mm diameter), frequent (5-20 pits/cm2), and regularly spaced. The metal surface in the pit is indicative of a chloride-driven pitting morphology. The key difference is that there is no free chlorine in the bulk water (an indicator that the chloramine is breaking down). The water chemistry log will have chloramine values above 3-4 ppm and pH below 7.0 or above 9.0 at different times. Testing the bulk water for free chlorine and chloramine will confirm the type of disinfection. The solution to these failures is to lower the chloramine concentration to less than 2.5-3.0 ppm or keep the pH closely at 7.5-8.5 or upgrade to duplex 2205 stainless steel. Treatment solutions for high chloramine systems If it is not possible to reduce the chloramine concentration to 3 ppm, the following procedures will lessen the risk of pitting: Electropolishing: Electropolished surface (Ra <0.2 µm) increases chloramine threshold by ~1 ppm. pH stability The pH should be maintained in the range 7.8 – 8.2 by adding a pH controller. Decomposes to free chlorine at low pH. Decomposes to NCl3 at high pH. Lower heater setpoint from 55C to 50C (40-50% reduction in chloramine decomposition rate) Alloy upgrade: Duplex stainless steel 2205 (higher pitting resistance equivalent number, PREN >35) or 316L with titanium nitride (TiN) inclusion control. Conclusion: 316 heater life @ 55°C pH and Chloramine control 316 stainless steel sheathed heaters in municipal hot water at 55°C show pitting start above around 3-4 ppm monochloramine at pH 7.5-8.5 in 1,000-3,000 hours and perforation in 2-5 years. As soon as ph drops below 7.0 chloramine breaks down to free chlorine and even 2-3 ppm chloramine is quite aggressive and will pit in 1-2 years. 316 sheaths with data on chlorine and pH and a pH range of 7.5-8.5 and monochloramine <2.5 ppm for 10 year life shall be specified by engineers for chloramine treated water. Electropolished 316L or duplex 2205 is suggested for chloramine levels over 3 ppm or where pH control is not reliable. The present method, connecting chloramine concentration and pH to measured pitting initiation times at 55°C, allows users to define water treatment programmes which prevent disinfectant-driven pitting in municipal hot water systems.