Desalinated water has a reputation problem it does not deserve — and a chemistry problem that project teams routinely underestimate. A seawater or brackish-water reverse-osmosis plant produces water that is unusually low in dissolved solids, low in alkalinity and often high in chloride. The result is a supply that is chemically aggressive to the very metals and elastomers used in valves, fittings and taps. In coastal and arid-region projects, this is now a mainstream specification problem.
Corrosion is rarely about a single number, but in desalinated supplies two do most of the damage: total dissolved solids and alkalinity. WHO's guidance *Desalination for Safe Water Supply* (2007) and the *Guidelines for Drinking-water Quality* both note that very low-TDS water is poorly buffered and can be corrosive and unpalatable, which is why utility practice remineralises RO permeate before distribution. Blended and partially remineralised product water, however, often still sits far below the mineral profile of conventional supplies.
The practical indices matter more than raw TDS. A Langelier Saturation Index (LSI) below about −0.3, or a Ryznar Stability Index above roughly 7.5, tells you the water wants to dissolve calcium carbonate — and that it will attack the protective films inside copper tube, brass bodies and cement-lined pipe. Calcium carbonate precipitation potential (CCPP) is the more useful design measure, commonly targeted at roughly +4 to +10 mg/L as CaCO₃. AWWA Manual M58, *Internal Corrosion Control in Water Distribution Systems*, remains the reference for balancing that equation.
Brass is a copper-zinc alloy, and in low-pH, high-chloride, low-flow water the zinc dissolves selectively out of the structure. The body loses strength, becomes spongy and copper-coloured, and eventually fails at a thread or stem. It appears first in hot, stagnant zones: dead legs, long-closed isolating valves and the top of fittings where velocity is near zero.
EN 12165 CW602N is the recognised dezincification-resistant (DZR) brass, verified by the EN ISO 6509 test, which measures how deep dezincification penetrates a sample. DZR grades are specified to hold that depth to a maximum of 200 µm. A general-purpose free-machining brass such as CuZn39Pb3, or any zinc die-cast component, should not be buried in an aggressive-water system.
Lead is the second consideration. NSF/ANSI/CAN 61 covers the health effects of wetted components, and NSF/ANSI/CAN 372 caps weighted-average lead content of wetted surfaces at 0.25%. In the UK the gate is WRAS approval through BS 6920 material testing, under the Water Supply (Water Fittings) Regulations 1999. The revised EU Drinking Water Directive (EU) 2020/2184 tightens the lead parametric value to 5 µg/L by January 2036. Low-TDS, low-pH water is precisely the chemistry that leaches lead from a marginal alloy.
Coastal and desalination-served supplies carry chloride, which drives pitting, crevice corrosion and, once temperature rises, stress-corrosion cracking in austenitic stainless steel. Type 304 is commonly limited to around 200 mg/L chloride in warm, stagnant service; above that the normal upgrade is Type 316/316L or a duplex grade. Chloride also accelerates dezincification and works with residual disinfectant to break down the passive film on brass.
Velocity is the other half of the story. High velocity strips protective films and causes erosion-corrosion; near-zero velocity concentrates chloride and lets biofilm establish. European guidance — BS EN 806-2 and BS 8558 — generally keeps velocities in copper and brass below roughly 1.5 m/s, and a desalinated system should be designed to that limit from the outset.
Body material gets the attention, but most first-year leaks in aggressive-water systems come from elastomers. EPDM is the default for hot and cold potable water because of its resistance to oxidising disinfectants, yet not all EPDM is equal: peroxide-cured, low-extractables grades cope better with chloraminated water, the residual many desalination networks use because it is more stable over long transmission distances. PTFE seats and seals in ball valves are the most chemically inert option and the right default where the water is aggressive. Three rules follow: no zinc die-cast parts on the wetted path; brass-to-steel transitions through dielectric unions; and no galvanised steel downstream of a soft or low-TDS supply, where it corrodes to white rust within months.
Inland arid projects present the opposite problem. Supplies are frequently high in hardness and TDS and delivered warm, so the dominant failure mode is scale: it blocks ports, jams thermostatic cartridges and blinds strainers. Where desalinated water is blended with a hard inland source across seasons, the design must satisfy the corrosive case first and manage scaling chemically, because pH adjustment during blending changes what the water does to metal.
Measure what is actually at the tap: pH, conductivity/TDS, temperature and chlorine on site, then send a laboratory sample for alkalinity, calcium hardness, chloride, sulfate, silica and lead. Calculate LSI, RSI and CCPP from the source analysis rather than assuming the desalination stage finished the job — the blend at the tap is what corrodes the metal. Look for copper-green staining or grey-white deposits on brass after descaling, and cut through one retired fitting at the threaded throat to check for the copper-coloured, spongy dezincified band. Map dead legs and low-flow branches too: aggressive water does its worst where nothing moves.
Aggressive water does not announce itself. It quietly removes zinc from a brass body, lead from a marginal alloy and the protective film from a copper tube, and the failure arrives months or years later, once the warranty conversation has started. Specifying DZR brass, the correct elastomer and a chloride-appropriate stainless grade turns a repeat site failure into a service life. The specification is cheap; the call-back is not.
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**About the author**
*Zhejiang Xindong Sanitary Ware Co., Ltd. — Technical & Export Team*
Xindong has manufactured brass valves, fittings, thermostatic shower valves and mixer taps in Yuhuan, Zhejiang for more than 25 years, from a 30,000 m² plant with 380+ employees, in-house design and testing laboratories, and over 65 design and structure patents. Products are developed and tested in line with the approval regimes of the company's export markets, including WRAS, TMV2/TMV3, cUPC and NSF/ANSI requirements, and are supplied to importers, MEP contractors and water-treatment integrators across Europe, the Middle East and North America.
Contact: No.242 JinHai Road, Xuanmen Industrial Zone, Yuhuan, Zhejiang, China · +86-0576-87499008 · [email protected] · https://www.cn-xindong.com
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The company, founded in 1999 and located in Yuhuan,specializes in taps, thermostatic shower valves and shower kits. With over 30,000 sqm of space and 380+ employees, we hold numerous certifications and 65+ patents.
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