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Scale & Corrosion Control Companies
Scale and corrosion control programs, inhibitors, monitoring, and side-stream treatment for cooling and boilers.
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Scaling and Corrosion Control in Water Systems: Langelier Index, Chemical Dosing, and Materials Selection
Scaling and corrosion are opposing water chemistry phenomena that both cause significant damage to water infrastructure. Scale formation occurs when mineral compounds exceed their solubility product (Ksp) and precipitate on pipe walls, heat exchanger surfaces, and membrane elements: calcium carbonate (CaCO3, calcite, Ksp 3.3 times 10 to the -9 at 25 degrees C) is the dominant scale in hard water systems; calcium sulphate (gypsum, Ksp 4.93 times 10 to the -5); barium sulphate (Ksp 1.1 times 10 to the -10, most problematic in oil and gas); silica (amorphous SiO2, solubility approximately 120 mg/L at 25 degrees C); iron (ferric hydroxide, iron bacteria). Corrosion occurs when metals dissolve electrochemically: galvanic corrosion at dissimilar metal junctions; general corrosion of cast iron at pH less than 6.5; dezincification of brass fittings in aggressive (low alkalinity) water; pitting corrosion of stainless steel in chloride-rich water (Cl- greater than 500 mg/L). The Langelier Saturation Index (LSI = pH - pHs) quantifies CaCO3 stability: LSI positive means scaling tendency; LSI negative means corrosive tendency; target LSI 0 to +0.5 for plumbing protection without significant scale formation.
Antiscalant chemicals inhibit scale formation in membrane systems (RO, NF, MF) and cooling water circuits. Antiscalant mechanisms: threshold inhibition (polyphosphates, phosphonates, polyacrylates at 2 to 10 mg/L adsorb on crystal nuclei, inhibiting CaCO3 precipitation at concentrations 50 to 200 times above solubility); crystal modification (distorting crystal growth to produce non-adherent sludge); dispersion (polyelectrolytes prevent crystal agglomeration). Common antiscalants: HEDP (1-Hydroxyethylidene-1,1-diphosphonic acid, phosphonate) effective for CaCO3, CaSO4, BaSO4; ATMP (aminotrimethylene phosphonic acid) for silica and iron; polyacrylate (MW 2,000 to 6,000) for CaCO3 and CaSO4 dispersion; maleic acid copolymers for mixed scaling. Antiscalant dosing: typically 2 to 10 mg/L for RO feed water; selection based on water chemistry analysis (Langelier, Stiff-Davis, Ryznar indices) and manufacturer Saturation Index modelling software (Lanxess Lewabrane, Dupont Water Solutions FilmTec WAVE, Avista Technologies Advisor). Over-dosing causes organic fouling on membranes; under-dosing causes scale fouling and irreversible flux decline.
Corrosion inhibitors for water distribution systems include orthophosphate dosing, which forms a passive protective layer on lead, copper, and iron pipe surfaces: orthophosphate at 0.5 to 1.0 mg/L as P (maintaining pH above 7.5) reduces lead plumbosolvency by 50 to 80 percent (UK DWI Action Level for lead: 10 ug/L per EU DWD 2020/2184; US EPA Action Level 15 ug/L per LCR). Polyphosphate (NaHexaPO4, sodium hexametaphosphate) forms a scale-resistant coating in distribution pipes but may complex with lead and increase plumbosolvency - use is restricted by DWI in UK for lead pipe networks. pH and alkalinity adjustment: lime (Ca(OH)2) or soda ash (Na2CO3) dosing raises pH and adds alkalinity (target alkalinity 50 to 100 mg/L as CaCO3, pH 7.5 to 8.5) for soft water catchments (Scotland, Wales, SW England) to minimise corrosion. UK DWI Guidance on Water Treatment Chemicals (2023) and US EPA 3T's guidance on lead control specify corrosion inhibitor requirements.
Frequently Asked Questions
What is the Langelier Saturation Index and how is it used?
The Langelier Saturation Index (LSI = pH - pHs) indicates whether water will precipitate or dissolve calcium carbonate (CaCO3). pHs is the theoretical pH at which the water is in exact equilibrium with CaCO3, calculated from: pHs = (9.3 + A + B) - (C + D), where A relates to TDS, B to temperature, C to calcium hardness, and D to total alkalinity (per Langelier's original 1936 formula). LSI greater than 0: water is supersaturated, scale-forming tendency; LSI equals 0: stable, no tendency; LSI less than 0: undersaturated, corrosive tendency. Practical targets: distribution systems: LSI 0 to +0.5 (slight scaling tendency protects pipe walls without excessive scale buildup); RO feed water: LSI less than 0 at the concentrate end (with antiscalant) to prevent CaCO3 scaling on membrane; cooling water: LSI 0 to +1.5 (higher scale tendency managed by antiscalant and blowdown). Related indices: Ryznar Stability Index (RSI = 2 times pHs - pH): RSI less than 6 means scale forming; RSI greater than 8 means corrosive. Stiff-Davis Index corrects for high ionic strength (seawater) where Langelier underestimates scale tendency.
How do antiscalants prevent scale in RO systems?
RO membrane scaling occurs in the concentrate stream where rejected ions accumulate to concentrations 4 to 8 times the feed (at recovery of 75 to 87 percent). Without antiscalant, CaCO3 scale forms at LSI greater than 0 at the concentrate, reducing membrane flux, increasing feed pressure, and potentially requiring acid cleaning (reducing membrane life). Antiscalant mechanism: (1) Threshold inhibition - antiscalant molecules (phosphonates, polyacrylates at 2 to 6 mg/L) adsorb onto CaCO3 crystal nuclei, blocking active growth sites; prevents precipitation at concentrations far above solubility limit; (2) Crystal modification - antiscalant distorts crystal habit, producing non-adherent, non-compacting sludge that is swept through the membrane by crossflow; (3) Dispersion - polymeric dispersants prevent crystal-to-crystal agglomeration. Selection: antiscalant is chosen by modelling concentrate chemistry using design software (Lanxess BWA, Dupont WAVE, Avista Advisor) - software predicts saturation indices for all potential scales (CaCO3, CaSO4, SrSO4, BaSO4, SiO2) at design recovery and recommends antiscalant type and dose. Silica scaling (amorphous SiO2) requires specific antiscalants effective at elevated pH.
How is lead corrosion controlled in drinking water distribution?
Lead plumbosolvency control is required where lead pipes, lead solder, or lead-containing brass fittings exist (pre-1970 housing stock in UK; pre-1986 in US). Control measures: (1) Orthophosphate dosing: 0.5 to 1.0 mg/L as P maintained in distribution; at pH 7.5 to 8.0, orthophosphate reacts with lead surface to form hydroxypyromorphite (Pb5(PO4)3OH, Ksp 10 to the -76.8), a very insoluble protective coating; reduces lead at tap from greater than 100 ug/L to less than 10 ug/L in most cases; (2) pH optimisation: maintaining pH 7.5 to 8.5 reduces lead solubility by 2 to 5 times; dose lime or soda ash for soft water catchments; (3) Target UK DWI Action Level: 10 ug/L Pb at consumer tap (per EU DWD 2020/2184, 10 ug/L by 2036, interim 25 ug/L by 2022); US EPA LCR (Lead and Copper Rule Revisions 2021): AL 15 ug/L, trigger level 10 ug/L; (4) Lead pipe replacement (LSL - Lead Service Line replacement): UK WICs funded programmes replacing lead pipes (Ofwat AMP8 investment); US EPA LCRR requires LSL inventory and replacement within 10 years.
What causes scale in heat exchangers and how is it removed?
Heat exchanger scaling causes include: (1) CaCO3 (most common): forms in hard water heated above 60 degrees C (solubility decreases with temperature; hot water storage vessels, boiler feedwater, cooling tower heat exchangers); deposition rate increases 2 to 3 times per 10 degrees C temperature rise above 40 degrees C; (2) Calcium sulphate: solubility retrograde above 40 degrees C (decreasing with temperature), problematic in flash desalination and geothermal heat exchangers; (3) Iron oxide/hydroxide: corrosion products from upstream pipework depositing on heat exchanger surfaces; (4) Biological fouling: biofilm formation at 20 to 45 degrees C on surfaces reducing heat transfer coefficient by 10 to 40 percent. Scale removal methods: (1) Chemical cleaning: circulate acid (5 to 15 percent HCl for CaCO3, EDTA for iron scales) at 20 to 40 degrees C for 4 to 8 hours; inhibited acids (Rodine, Hakemull) prevent excessive base metal attack; neutralise and rinse before restart; (2) Mechanical cleaning: high-pressure water jetting (300 to 700 bar), shot blasting, or rotary brush for severe deposits; (3) Electrolytic descaling: impressed current reversal; (4) Prevention: antiscalant dosing (threshold inhibition), water softening of make-up water (cooling towers), temperature control, blowdown management (Langelier control).
A pharmaceutical manufacturing facility in the East of England was losing 18% of heat exchanger capacity annually due to calcium carbonate scale deposition in its purified water generation circuit (feed water hardness 320 mg/L as CaCO3, final heating stage to 80 degrees C). Chemical descaling downtime cost GBP 95,000 per year in lost production and labour.
A water treatment contractor installed a twin-vessel sodium cation exchange softener on the heat exchanger feed circuit, reducing hardness to below 2 mg/L as CaCO3. Corrosion inhibition was upgraded from chromate to a COSHH-compliant phosphonate/azole blend dosed at 120 mg/L. A quarterly inhibitor monitoring programme with coupon racks confirmed film persistence. The softener regeneration brine was neutralised and discharged under the existing trade effluent consent.
Scale deposition rate fell by 96% in the first year, measured by coupon analysis. Heat exchanger cleaning frequency dropped from quarterly to once per year. Lost production from scale-related downtime was eliminated, saving GBP 88,000 per year. Corrosion rate on mild steel coupons fell from 0.12 mm/year to 0.02 mm/year, extending asset life projections by 12 years.
Questions to Ask Shortlisted Providers
- 1
What is the feed water hardness, alkalinity, pH, temperature, and Langelier Saturation Index (LSI)?
LSI below 0 indicates corrosive tendency; LSI above +0.5 indicates scaling tendency; LSI and Ryznar Stability Index are the primary design inputs for chemical treatment programme selection.
- 2
What materials of construction are present in the system (mild steel, stainless steel, copper alloys, galvanised pipework)?
Multi-metal systems require inhibitor programmes protective of all metals simultaneously; copper-aluminium combinations create galvanic corrosion risks; galvanised pipework requires alkalinity management to prevent zinc release.
- 3
What is the current blowdown rate and cycles of concentration in open cooling systems?
Operating above recommended cycles of concentration without adequate biocide and inhibitor programmes causes accelerated scale and Legionella risk; ACOP L8 requires documented cooling tower water treatment regimes.
- 4
What are the discharge consent conditions for spent regeneration brines or blowdown from treatment systems?
Ion exchange regeneration brines (high chloride, TDS) may require dilution before discharge; limits are set by the water company in the trade effluent consent under WIA 1991 Section 118.
- 5
Is there a current Legionella risk assessment and written control scheme covering all open cooling systems and hot water services?
ACOP L8 requires a Legionella risk assessment for all premises with cooling towers or hot water systems; scale and corrosion in these systems directly increases Legionella colonisation risk and is a key control measure in the written scheme.
What Drives Cost in This Category
Industrial inhibitor programmes typically cost GBP 0.05 to 0.25 per m3 of treated water; cooling tower programmes cost GBP 8,000 to 60,000 per year depending on system volume and make-up rate.
Twin-vessel softeners (500 to 5,000 L/hr) cost GBP 8,000 to 45,000 installed; salt costs run GBP 0.15 to 0.40 per m3 of softened water; brine disposal may add GBP 0.05 to 0.15 per m3.
Quarterly water analysis and coupon inspection costs GBP 3,000 to 12,000 per year per system; online corrosion monitoring probes add GBP 5,000 to 20,000 capital but reduce service visits.
Chemical descaling of a heat exchanger train costs GBP 5,000 to 30,000 including inhibited acid, labour, neutralisation, and effluent disposal; production downtime cost typically exceeds chemical cost by 3 to 10 times.
Key Regulations & Standards
Mandatory Approved Code of Practice for control of Legionella in water systems; requires Legionella risk assessment, written control scheme, monitoring records, and competent person appointment for all open cooling towers and hot water services; scale and corrosion control are named control measures.
Scale and corrosion inhibitors (chromates, biocides, phosphonates) are hazardous substances requiring COSHH assessment, MSDS review, PPE specification, and emergency procedures; chromate-based inhibitors are restricted under REACH Regulation (EC) No 1907/2006 Annex XIV.
Discharge of ion exchange regeneration brines, acidic descaling effluent, and cooling tower blowdown to sewer requires a Trade Effluent Consent; consent conditions specify volume, pH, temperature, and chemical concentration limits.
Specify methods for alkalinity, hardness, pH, and conductivity measurement used to calculate Langelier Saturation Index and monitor scaling/corrosion control programme effectiveness; laboratories performing these tests should be UKAS-accredited.

















