Effect of H2S on corrosion in polluted waters: a review

Effect of H2S on corrosion in polluted waters: a review M. S. Wiener, B. V. Salas*, M. Quintero-Nu´n˜ez and R. Zlatev The present article provides an overview of the effect of hydrogen sulphide (H2S) on corrosion in polluted waters, including sea, river, brackish, geothermal and sewage waters. H2S is a weak, reducing acid which originates from sulphide minerals by natural acidification and/or from sulphur – bearing, decaying organic matter by bacterial action. Human and industrial activities increase the generation of corrosive gases, dissolved in water: CO2, H2S and NH3. Carbon steel, stainless steel, aluminium alloys and copper base alloys are corroded by H2S, producing metallic, nonstable sulphide films. The H2S content in various waters, the electrode potential pH (Pourbaix) diagrams for Fe and Cu in H2S containing systems, and the electrochemical and corrosion performance of steel in oxygen depleted, H2S polluted waters, which indicate active corrosion behaviour, are presented and discussed. Keywords: Pollution, Corrosion, Waters, Hydrogen sulphide, Steel

Introduction There is deep universal concern today about the influence of pollutants on the environment including soil, air and in particular water, and about their effects on the durability of engineering materials and the deterioration of structures and the infrastructure. Water pollutants affect the terrestrial, atmospheric and aquatic environments, and even when present at very low levels of a few ppm may (e.g. in the case of heavy metals) impair human health, aquatic life and water quality. The avoidance of water pollution is, therefore, an important part of water resource management. There is a direct relationship between prevention and control of corrosion of the infrastructure and the protection and preservation of the quality of the environment. The water infrastructure system, including water production and treatment facilities, transmission and distribution pipelines and the sewer system, is subject to corrosion with an annual cost, in the USA alone, of $36 billion.1 Corrosion in water is affected by the presence, solubility and activity of four gases: oxygen (O2), carbon dioxide (CO2), H2S and ammonia (NH3). Human and industrial activities increase the generation of CO2, H2S and NH3. The sulphide ion (S22) has a tremendous affinity for many heavy metals. Sediments in polluted marine and fluvial ports may contain insoluble sulphides, e.g. FeS, which have to be removed periodically by dredging so that the soil can be put right and restored.2 In contrast, clean, natural sea, river, lake, gulf and estuary waters do not contain H2S and/or soluble

Red Nacional de Corrosio´n, Laboratorio de Materiales y Corrosio´n, Instituto de Ingenierı´a, Universidad Auto´noma de Baja California, Blvd. Benito Jua´rez, s/n, C.P. 21280, Mexicali, Baja California, Me´xico *Corresponding author, email [email protected]

ß 2006 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 16 February 2006 DOI 10.1179/174327806X132204

sulphides. The H2S contents of the various types of contaminated waters under the different conditions considered herein are given in Table 1. The pollution of freshwater bodies, such as rivers, is not a recent phenomenon. The prohibition of discharges into rivers of stinking water (which may contain H2S) from tanneries is mentioned in the ‘Mishna’, a classic Jewish legal compendium from the second century AD. The present paper deals in particular with the influence of H2S on corrosion in polluted aqueous systems.

H2S enhanced corrosion H2S is a colourless gas with an offensive odour suggestive of bad eggs. It is a weak reducing acid, readily soluble in water with the following ionic dissociation H2 S~Hz zHS{ ~2Hz zS2{

(1)

While ever present in anaerobic sea water, studies have found that hydrogen sulphide is also widely distributed in aerobic surface waters,3–5 albeit at 4–6 orders of magnitude lower concentrations than those in anoxic regions. In both anaerobic and aerobic waters, hydrogen sulphide exists as a dissolved gas [H2S(g)], its dissociated ions, bisulphide (HS2) and sulphide (S22), and as dissolved metal–sulphide complexes. The term ‘free sulphide’ is the sum of the uncomplex species [H2S(g)zHS2z S22], while ‘complex sulphide’ includes all metal–sulphide species; the total dissolved sulphide is the sum of the free and complex sulphides. Sulphide pollution of sea water in coastal areas can occur owing to industrial waste discharge, either from biological and bacteriological processes in sea water (seaweed, marine organisms or microorganisms, including the sulphate reducing bacteria) or from disturbed anoxic coastal sediments.6,7

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The sulphide related bacterial activity can be expressed by the following reactions: H2S is oxidised to H2SO4 by sulphide oxidising bacteria (SOB) such as thiobacillus and thioxidans H2 Sz2O2 ~H2 SO4

(2)

Sulphates are reduced to sulphide by sulphate reducing bacteria (SRB) such as Desulfovibrio desulfuricans 2{ SO2{ z4H2 O 4 z4H2 S

(3)

Bacteria form a biological film or biofilm on the metal surface, influencing corrosion activity through their ability to change the water properties beside the metal: pH, ionic strength, dissolved oxygen (DO) content, oxidising power, temperature, flow velocity and concentration. The chemistry of the metal/water interface under the biofilm is altered, inducing localised corrosion. H2S producing bacteria also operate in the human body; the smell of flatulence and bad breath is largely due to trace amounts of the gas.8 Sulphide combines with dental alloys containing Cu and Ag forming low solubility products Ag2S and CuS that constitute tarnish films.9 It has been widely reported that sulphides are often present on alloy surfaces as the result of contact with polluted waters. Extensive research has shown that corrosion is particularly aggressive in polluted harbours and estuaries, where the ebb and flow of the tides cause a change from polluted and anaerobic to relatively aerated sea water.6,10–14 When shifting between polluted and non-polluted environments, it was concluded that the sulphide and the oxygen can coexist for a short period and that the accelerated attack was not caused by the presence of sulphide alone but by the simultaneous presence of sulphide and oxygen at the metal surface. Syrett11 has suggested that the sulphide ion (or sulphide oxidation products) can interfere with the normal growth of the protective oxide film that forms on the surface of copper based alloys exposed to sea water. This results in the production of porous, non-protective cuprous sulphide films which interfere with the normal passivation of the alloys when exposed to unpolluted aerated sea water. This corrosion activity is evident in FezH2 S~FeSzH2

(4)

Fe2z zS2{ ~FeS

(5)

CuzH2 S~CuSzH2

(6)

2CuzH2 S~Cu2 SzH2

(7)

Sea

Level, ppm Location/source

3–4 50 River 1–4 50–100 Brackish 0.1–6 Thermal springs 8–25 Geothermal 35–190 0.3–15 3–20 0.6–7.7 Sewage 2–7

222

Ref.

San Diego Harbour Polluted estuarine water Kishon River Anthropogenic pollution Dead Sea Basin Dead Sea Steam, Wairakei wells Steam condensates Deep wells Field wells, Mexico Municipal

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FeSz2Hz ~Fe2z zH2 S

(8)

The formation of black metallic sulphide suspensions and/or deposits is characteristic of H2S induced corrosion. H2S penetrates the lattice of some steels, makes them brittle and causes stress corrosion cracking (SCC) failure in particular of high strength steel.15 Sulphide SCC is induced, in an acidified 5%NaCl solution saturated with H2S, in an evaluation test covered by the National Association of Corrosion Engineers (NACE) Standard TM0177.16 According to Sanchez and Schiffrin17 the main effect of the pollutant is drastically to increase the rate of the oxygen reduction reaction while the anodic process remains largely unaltered. The chemistry of aqueous sulphides and oxidation products is extremely complex but available data indicate that dissolved sulphide is rapidly oxidised in both sodium chloride and sea water solutions. Ostlund and Alexander18 showed that for air saturated sea water with a sulphide concentration of 3.8 ppm, the halflife of the sulphide was of the order of 20 min. Avrahami19 in a detailed mechanistic study of reactions showed that elemental sulphite (SO322), sulphate (SO422) and thiosulphate (S2O322) are formed and the oxidation of sulphur to the oxyanions is slow. Also sulphides may be oxidised by dissolved oxygen to a variety of polysulphides.

H2S toxicity It is appropriate to report in the context of the present paper the toxicity of H2S since this also affects quality of the environment and human health, central issues of modern society. H2S gas emitted into the atmosphere from municipal sewage or polluted rivers causes inflammation of the eyes, skin burns and respiratory diseases such as rhinitis, bronchitis and pneumonia. When inhaled in small amounts, the gas produces headaches and nausea; a large amount produces paralysis. H2S is very toxic, rapid death ensues from exposure to air containing .1000 ppm H2S owing to asphyxiation from respiratory system paralysis. Lower doses cause dizziness and excitement because of damage to the central nervous system. Cases of the death of workers following the release of H2S from sewage installations and from plant for the removal of sulphur from natural gas have been reported.20,21

Sea water The major chemical constituents of sea water are quite consistent worldwide15,22. Its minor components include dissolved trace elements, organic nutrients and the dis solved gases (O2, CO2 and NH3) and H2S in contaminated sea water. These are the principal corrosion rate controlling factors of the fixed and mobile elements of the marine infrastructure.2 In recent decades, anthropogenic pollutants have penetrated the oceans, the seas and marine gulfs, in particular in coastal areas. Concentrations of the minor constituents are radically changed by pollution. H2S is the most critical pollutant in sea water, and may reach levels of 50 ppm or more in severely polluted coastal or harbour water. Noted by its foul smell, H2S is produced from sulphur bearing

Table 1 Sulphide levels in polluted waters Water

In acidic waters H2S is formed again and this process is repeated, enhancing corrosion

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A river that empties into the sea, near a large city and a marine port, belongs to a coastal ecosystem often including industrial parks and agricultural fields. The Kishon River and port, near the City of Haifa in the north of Israel, on the Mediterranean Sea, is a typical system suffering from corrosion enhanced by anthropogenic pollutants, particularly H2S (Ref. 25). The Kishon port includes docks for both fishing and general cargo ships, a marina harbour for yachts and rowing boats, and a shipyard for the construction and repair of vessels. The Kishon River crosses agricultural fields and an industrial region and collects the wastes generated by these activities. What is more, the treatment plant for processing municipal wastewater sometimes discharges non-treated, excess waste into the river flowing into the port and Haifa Bay. Pollution of the river and port water by acidic industrial effluents and by the biological decomposition of organic compounds present in these agricultural and municipal wastes has caused corrosion damage to fishing boats and pleasure boats, port installations and steel floating dock equipment.2 During periods of intensive disposal of acidic industrial effluents, the pH value of the river water has fallen to values in the range 2–4, the sulphide concentration being 1–4 mg L21 and the iron ion content being 1–3 mg L21, indicating an acidic medium. An intense process of river rehabilitation by controlling thedischarge of effluents and cleaning the river bed by dredging has decreased the content of suspended and deposited organic matter, diminishing the intensity of pollution and corrosion, as reported by the Kishon River Authority.26

decaying organic matter by bacterial activity. Coastal areas and harbours beside large cities suffer from depressed dissolved oxygen levels because of biological oxygen demand, which is dependent on the seasonal variation of temperature. Pitting corrosion of low carbon steel in the polluted sea water of the San Diego harbour (CA, USA) was several times higher than the corrosion usually experienced in clean sea water.15 In sea water polluted with 3–4 ppm H2S, unstable sulphide films form on steel and copper alloys, which accelerate corrosion since they are cathodic to the bare steel or copper surface. The water below the surface layer is frequently anaerobic; sometimes microbiological oxidation of sulphur in algal proteins is responsible for the local acidification of sea water. The mud on the seabed in coastal regions contains anaerobic bacteria that develop H2S which can attack steels and copper alloys at relatively low corrosion rates owing to the reduced supply of oxygen. The Gulf of Mexico is part of the Atlantic Ocean. It is a large expanse of salt water; several American and Mexican commercial ports are located along its shores and many rivers empty their waters into it, e.g. the Rio Grande, the Coatzacoalcos, the Grijalva and the Mississipi. The effluents from these regions drain into the Gulf of Mexico contributing urban, agricultural and industrial wastes, resulting in either hypoxic conditions (when the level of dissolved oxygen falls below 2 mg L21) or anoxia (when it falls to zero). Nutrients delivered to estuarine and coastal systems support biological activity that depletes oxygen. The Gulf of Mexico is one of the largest oil producing areas in the world; high levels of dissolved and dispersed hydrocarbons are encountered in its waters. These oil traces contain H2S which also impacts on the oxygen content of the water.23 There is a significant difference between the concentrations of the major constituents of sea water and river water, as shown by their typical compositions (Table 2).15 The low salt concentration in river water means that the corrosive pollutants play a more major role in determining the extent of corrosion in river water than in sea water. The rivers, inland streams and lakes are highly dynamic freshwater systems, often with continuous turbulent flow. They are subjected to the input and loss of a variety of materials from both natural and anthropogenic sources which undergo chemical and biological processes such as hydrolysis, precipitation with the formation of sludge on the river bed, aggregation of suspended colloidal particles, oxidation–reduction reactions, and bacterially influenced processes such as putrefaction, decomposition, fermentation, respiration, etc. Dissolved H2S in raw water, often present in high iron content water and low pH water, tarnishes silverware and stains metallic bathroom fixtures. Water with .6 ppm H2S requires chlorination followed by filtration.24

Brackish waters An accepted classification of saline waters according to ASTM standard D1129 encompasses sea water, brackish water and concentrated brine. Brackish water from wells in the Dead Sea Basin in the Judah Desert, Israel, contains 2–5 g L21 total dissolved solids (TDS).27 The content of dissolved gases (in ppm) varies as follows: H2S, 1–6; O2, 1–3 and CO2, 10–40. A high level of H2S implies a low content of O2, since it oxidises H2S to elemental sulphur which appears in the soil of the oxidation ponds H2 Sz0:5O2 ~S0 zH2 O

(9)

Some sulphur rich, thermal spring waters flowing into the Dead Sea contain 60–210 g L21 TDS and 8–25 ppm H2S. In recent decades, brackish water has been used, after pretreatment and H2S removal, as feed water for desalination plants.28 The main source of H2S in underground brackish water is acidic attack on pyrite and other mineral sulphides by organic acids, e.g. humic, fulvic and carbonic acids. H2S is responsible for lowering the pH of brackish water and for an acidic corrosion mechanism with cathodic depolarisation and increased

Table 2 Sea water and river water composition Component

Naz

Kz

Mg2z

Ca2z

Cl2

HCO32

SO422

Si(OH)4

Salinity, %

Sea, g kg21 River, mg L21

10.7 6.3

0.4 2.3

1.3 4.1

0.41 15.0

19.3 7.8

0.14 58.4

2.71 11.2

----20.9

3.5 ----

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anodic dissolution. Other species encountered in brackish waters, namely Cl2, CO2, HCO32, H2S, HS2 and SO422 affect their electrical conductivity and corrosivity. The pH value of brackish water is generally in the range 4.5–7.5, depending on the levels of H2S and CO2 and the nature and amount of acidic or alkaline salts. A series of corrosion rate measurements on carbon steel were carried out by adding Na2S and lowering the pH with HCl to simulate the conditions in slightly acidic sulphide polluted water. The corrosion potential Ecorr was measured according to ASTM standard G3 and the redox potential Ered of the water was determined applying ASTM standard D1498. The corrosion rate was calculated from the polarisation plots by Tafel slope extrapolation. The results displayed in Table 3 show that H2S, a reducing acid, causes a drastic shift of the redox potential Ered towards more electronegative values from 0.30 to 20.19 V. On the other hand, the Ecorr values for active, corroding carbon steel are similar to those for all brackish water wells. Table 3 shows clearly the increase in corrosion rate with increasing H2S content. Brackish water from the Dead Sea Basin is utilised as industrial water in chemical plants producing fertilisers, chemicals and salts from the Dead Sea concentrated saline water. As it flows in steel or plastic pipelines, H2S gas is released from the water and escapes through a system of vents or is consumed by reaction with dissolved oxygen and/or scrap iron. When the water reaches the reservoirs, it is aerated by running it through perforated pipes which form a fountain system sending jets of water into the air. H2S is oxidised to elemental sulphur, recognised by its typical yellow colour, which is deposited in the reservoir soil, covered by geotextile sheets. In asbestos–cement pipelines, partially filled with flowing brackish water owing to the terrain topography, H2S gas accumulates in the upper (empty) part of the wet pipe, forming an acidic solution of H2S and H2SO4. Acidic attack of the alkaline components of the asbestos cement pipe occurs, resulting in significant loss of wall thickness and consequent loss of strength, sometimes with the formation of cracks and holes. On the other hand, full wall thickness is maintained in the lower part, which always remains covered with brackish water. The industrial water delivered to the production plants is practically H2S free and is saturated with oxygen.

Geothermal waters Geothermal energy – a form of renewable energy – is extracted from the heat carried by subterranean waters

1 Pourbaix diagram of electrode potential versus pH for iron in typical geothermal steam containing H2S

close to the earth’s surface. When the hot water is extracted it is partially or totally converted into steam, which is used to generate electricity or for other purposes such as district heating. Geothermoelectric power plants are located in many countries worldwide. Mexico occupies the third place in the geothermal energy generation league; its largest plant is situated in the Cerro Prieto area in the state of Baja California, and produces 720 MW. In order to harness the energy from the geothermal reservoir, the fluid has to be transported to the surface through well casings. Once the two phase geothermal fluid reaches the surface, it separates into geothermal steam and residual brine. The main contaminants of the geothermal fluid are H2S, NaCl and SiO2, which give rise to corrosion products, deposits and scale in the wells and casings. Apart from H2S, other corrosive species present are O2, CO2, Cl2 and NH3. The tendency to form non-protective scale is influenced by the H2S content of the fluid (Table 1). The corrosion reactions and corrosion products for iron in typical geothermal steam containing H2S are displayed in the Pourbaix diagram of Fig. 1, which indicates domains of immunity, corrosion and passivity as a function of

Table 3 Influence of added H2S on electrochemical parameters of carbon steel in brackish waters of Dead Sea Basin brackish waters

224

Well

TDS, g L21

H2S, ppm

pH

Ered, V (SCE)

Ecorr, V (SCE)

A

4.11

B

2.40

C

4.15

0.1 10 10 0.1 10 10 0.1 10 10

7.2 7.2 6.1 7.6 5.0 3.0 7.4 5.0 3.0

0.30 20.19 20.19 0.59 0.28 0.09 0.61 0.13 20.04

20.58 20.55 20.65 20.62 20.64 20.64 20.59 20.66 20.62

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Corrosion rate, mm year21 0.8 2.9 3.6 1.0 1.2 3.5 0.2 1.0 1.4

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potential and pH. Figure 2 presents the Pourbaix diagram for Cu, indicating areas for the formation of copper sulphides and oxides. At higher potentials and temperatures, when steam comes into contact with natural sulphur, the following oxidation reactions occur, producing ‘acidic sulphate’ steam H2 Sz2O2 ~H2 SO4

(10)

2S0 z3O2 z2H2 O~2H2 SO4

(11)

Massive concrete structures such as canals, pipeline supports and steam noise silencers are degraded by H2S gas trapped in condensed steam that permeates through the pores and cracks of the concrete wall, corroding the steel reinforcement. Furthermore, H2S is actively involved in microbially influenced corrosion (MIC) which develops in the geothermal environment. Corrosion control of carbon steel is achieved in Wairakei steam condensate by the formation of a stable magnetite (Fe3O4) scale with significant uptake of suspended fine silica (SiO2) that blocks the pores in the scale, thereby limiting the corrosion rate.29 The average redox potential of deep geothermal water at 250uC is 20.55 V versus SCE, indicating the presence of H2S generated by natural acidification of pyrite (FeS). There is a wide variety in the H2S content of steam from geothermal wells; for instance, at Wairakei the range is 35–190 mg L21. In deep well water, the H2S concentration is 3–20 mg L21. Several geothermal wells in Matsukawa, Japan, produced slightly acidic waters with pH values ranging from 3.7 to 5.5. Sulphide corrosion attack on steel takes place with complex mechanisms of

2 Pourbaix diagram of electrode potential versus pH for system Cu–S–CO2–H2O at 25uC

Effect of H 2 S on corrosion in polluted waters

corrosion and scale formation.30 The H2S content in the well water of several geothermal fields in Mexico varies from 0.6 to 7.7 ppm.31 Steam from the Cerro Prieto wells contains CO2, NH3 and H2S, the latter at a concentration of 5–6 ppm. Three types of scale form in these wells: calcite (CaCO3), amorphous silica (SiO2) and metallic sulphides, mainly of iron, lead and copper, which deposit at great depth as a result of the hot water–rock reaction. Serious corrosion problems arise from the attack of steel well casings associated with the presence of H2S and chloride salts.32 Scaling by calcite and aragonite and SCC in Cu and Ni alloys induced by H2S and Cl2 were found at the Kebili (Tunisia) geothermal field,33 owing to chemical changes resulting from degassing, evaporation and cooling processes used to produce water for oasis irrigation. Corrosion in a cooling tower and auxiliary equipment, promoted by the presence of H2S and calcite scale, has been encountered in the geothermal well fluids of the Olkaria plant in Kenya.34 The content of H2S in different condensate fluids varied from 0.2 to 1.4 ppm. The results of corrosion rate measurements, using a weight loss technique in cold and hot condensate, indicate that corrosion rates in acidic condensates are higher than those in neutral and alkaline condensates (Fig. 3). The Cerro Prieto geothermal wells emit H2S into the atmosphere surrounding the fields and the power plants. Other H2S emissions came from the plant chimney stacks, vapour ducts, noise silencers and cooling towers, totalling 22 740 t during 2000.35,36 Since the production capacity of Cerro Prieto has not changed over the last 5 years or so, it may be estimated that the same amount of H2S is still emitted today. The H2S concentration in the atmosphere around the geothermal fields ranges from 10 to 30 ppb. Typical ranges of natural and anthropogenic H2S under outdoor and indoor conditions are 0.7– 24 ppb and 0.1–0.7 ppb respectively.37

3 Corrosion rates of mild steel in condensate

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When H2S with other gases, e.g. O2, CO2 and HCl, settles on the wet surface of metallic structures, active corrosion ensues, forming non-protective, hygroscopic corrosion products. The impact of H2S in the geothermal fields in the south of California, near the Mexico–USA border, is considerably less than that in Cerro Prieto owing to the lower level of power production and the reinjection of the geothermal fluids into the wells.38

Sanitary sewage20,39 Municipal wastewater systems include pipes of different diameters up to 3 m, manholes and pumps. The generation of H2S from sulphur bearing organic waste and its subsequent oxidation to H2SO4 with the assistance of sulphate reducing bacteria, have led to microbially inflenced corrosion in metallic (steel, ductile and cast iron) and non-metallic (asbestos–cement and reinforced concrete) pipes. Localised corrosion attack, in particular in air pockets in the upper part of a pipe, occurs at H2S concentrations of 0.5–5.0 ppm in flowing sewage. Cementitious or polymer coatings might minimise H2S and microbially influenced corrosion in sewers. Materials with better resistance to these forms of corrosion, e.g. plastic piping, concrete pipe produced with type V cement displaying high sulphate resistance (ASTM standard C150) and ductile iron with increased carbon and silicon contents, will assure long lasting life for sewage pipelines. An attack by microbiologically generated H2S was detected in the reinforced concrete pipelines of the sewage system in the city of Tijuana, Mexico, causing burst and rupture of the pipes. The main H2S related bacteria were Desulfovibrio salexigens and other sulphate reducing bacteria.40 Hydrogen peroxide can be used to control odours and corrosion within wastewater collection systems by direct oxidation of the H2S H2 SzH2 O2 ~S0 z2H2 O

(12)

21

Doses of 5–10 mg L are effective for the avoidance of H2Sinduced corrosion in domestic wastewater.24 In alkaline waters at pH 9, metallic copper in contact with S22 ions forms thin films of copper sulphides, together with copper oxides.41

Concluding remarks Although many factors affect corrosion in polluted, oxygen depleted water, H2S is a dominant one. Its direct and indirect actions on the engineering materials, structures and environments can produce unstable metallic sulphides and significant changes in the chemistry of the water and sediments. Environments affected by H2S produced by bacterial activity may be treated with biocides at very low concentrations that kill the micro-organisms suspended in the liquid phase. Three main types of biocides can be applied: protein denaturants, oxidising chemicals and surface active agents. Typical biocides for dugout water include hydrogen peroxide, chlorine, hypochlorite, chlorine dioxide, bromine and ozone. Municipal facilities, industrial plants and agricultural fields should stop discharging their non-treated effluents into the seas,

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rivers and lakes of their regions, to prevent and control corrosion of the infrastructure in natural waters.

Acknowledgements The authors are grateful to the Kishon River Authority, Haifa, Israel for the supply of information on the pollutants in the waters of the Kishon River and port. The authors also thank the Programa de Corrosion del Golfo de Mexico for providing data on pollution and corrosion in the Gulf of Mexico and for the useful discussions held. The information about corrosion control in the Rio Colorado-Tijuana Aqueduct offered by COSAE-Comision de Servicios de Agua del Estado de Baja California, Mexico, is acknowledged, too.

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