CORROSION MONITORING OF SWCC PLANTS - I:1 Vapor-Side ...

CORROSION MONITORING OF SWCC PLANTS - I: 1 

Vapor-Side Corrosion Monitoring In Al-Khafji Desalination Plant 

Nausha Asrar, Anees U. Malik and Shahreer Ahmed 

Research And Development Center 

Saline Water Conversion Corporation 

P. O. Box 8328, Al-Jubail 31951, Kingdom of Saudi Arabia 

Madeeh Al-Khalidi and Khalid Al-Moaili 

Al-Khafji Desalination Plant, SWCC, 

Al-Khafji, Saudi Arabia 

SUMMARY 

Vapor-side corrosion is a major problem in both acid and additive dosed Multi Stage 

Flash (MSF) plants. With additive treatment, CO 2 is liberated in the evaporator by 

thermal decomposition of the bicarbonate ions. Besides CO 2 , O 2 and other glass like 

H 2 S, NH 3 and Cl 2 are also present. These gases are quite corrosive. The corrosion 

problem in vapor-side is aggravated due to uncertainties in concentrations of oxygen, 

carbon dioxide and other non-condensable gases. In such a situation, vapor-side 

corrosion could be a limiting factor in an MSF plant life if venting system is not 

designed properly. 

Under an approved research project on Corrosion Monitoring of SWCC Plants, 

Research and Development Center carried out corrosion monitoring at Al-Khafji plant. 

The firs report issued in 1994 dealt with the corrosion monitoring of evaporators in 

flashing brine. This report presents the results of the vapor phase corrosion monitoring 

in the Al-Khafji plant under the above mentioned project. 

Vapor phase corrosion monitoring at Al-Khafji desalination plant was carried out by 

studying the corrosion behavior of Cu-Ni 70/30 alloy, carbon steel and SS316L. The 

metallic coupons were fixed in vent lines and flash chambers exposed for 2,000 to 9,000 

hours and corrosion rates have been determined by weight loss method. The 

1 Issued as Technical Report N0. TR3804/APP91001 in July 1997. 

Part of this report was presented in Second Acquired Experience Symposium on Desalination Plants 

O&M, SWCC, Al-Jubail, Sept.29-Oct.3, 1997. 

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morphology of the scales and composition of the corrosion products were determined by 

scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX). 

The results of the vapor phase corrosion monitoring study indicate that cupronickel 

alloys are significantly corroded in vent line by S-containing compounds including H 2 S, 

whereas carbon steel appears to be less prone to corrosion due to sulfur. Stainless steel 

has been found to be most resistant to vapor side corrosion in flash chambers or vent 

lines. The higher corrosion rates in the middle stages (11-13) have been explained on 

the basis of combined effect of temperature, air in-leakage and accumulation of gases 

due to cascading. 

1. INTRODUCTION 

SWCC manages seawater desalination plants with a total capacity of 520 MIGD of 

potable water to meet the drinking water requirements of 70% population of the 

Kingdom. Some new plants are under construction to increase the capacity to 800 

MIGD [SWCC Annual Report 1994]. Failures of components may result in 

unscheduled shutdown of the plant and consequently in the reduced production of water. 

Multistage flash (MSF) evaporation and reverse osmosis (RO) are the two main 

techniques used for producing potable water from seawater. The former is 

predominantly employed for the production of desalinated water. An MSF plant is 

much more prone to corrosion due to its operation at higher temperatures. Corrosion 

monitoring of the component/material in an operating desalination plant is a profoundly 

important task because it provides a wealth of information about the condition of plant 

in terms of construction materials, components and operational parameters. All these 

informations can be useful in evaluating the performance and behavior of materials and 

efficiency of the plant. 

A number of papers appeared in recent years concerning with the corrosion monitoring 

of processing plants including chemical, petrochemical and desalination plants and also 

dealing with monitoring of pipe lines and cathodic protection systems. Cigna et al. 

[1985] have reported the continuous corrosion monitoring in an acid treated multi flash 

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desalination plant using linear polarization resistance (LPR) technique. The analysis of 

the tests data indicates that the on-line monitoring system could be utilized for 

foreseeing as an early warning for pH and oxygen upset for acid treated plants. 

Accordingly, the technique could be used to control oxygen level in brine and noncondensable 

carbon dioxide which are important parameters for efficient operation of 

the plant. Oldfield and Todd [1979] have studied theoretically the effect of oxygen 

content, flow rate and temperature on the corrosion of carbon steel, stainless steel 316 L 

and 90/10 cupronickel in seawater over the range of conditions likely to be met in a 

multi-stage flash distillation plant. The corrosion rate data from this study were found 

to be higher than the measured values presumably because of scale depositions. Some 

of the conclusions from this study are summarized in Table 1. Sato and Hamada [1969] 

stated that alkaline scale could be prevented in heat exchanger tubes and other parts of 

flash evaporators by keeping the brine pH below 7.5. CaSO 4 scales were not observed at 

twice concentrated seawater brine up to a temperature of 115 o C. Corrosion tests of a 

range of materials including welded joints were carried out by placing in the flash 

chambers of three MSF plants [Hodgkiess et. al. 1983]. The results of the plant studies 

have correlated well with laboratory studies indicating that accelerated corrosion of the 

welds in carbon steel plates is associated with relatively high oxygen concentration in 

the brine. It was observed that corrosion problems were not serious with properly-made 

welds between stainless steel plates. Nordin [1983] carried out a study in an air ejector 

system of an MSF plant using 316L and 904L stainless steel pipes, whilst 316L failed by 

SCC, the later grade appeared to be quite resistant to corrosion. Failure of SS316L was 

attributed to the presence of bromine in the venting gases. In a recent paper [Narain 

1992], corrosion problems experienced in a low temperature (max. distillation 

temperature : 59 o C) desalination plant, based on multi-effect evaporation supplemented 

with ejector vapor compression process are described. It has been found that severe 

crevice corrosion occurred at the joints of titanium heat exchanger tubes and 904L tube 

sheets; cathodic protection using iron anodes appeared to be effective in preventing 

corrosion. Aluminium brass tubes suffered impingement corrosion and were replaced 

with 90/10 Cu-Ni tubes. Erosion-Corrosion of 90/10 Cu-Ni ejector condensers was 

prevented by replacement with alloy 254 SMO. 

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A survey of some important work carried out on corrosion monitoring in MSF plant as 

cited above indicates that the corrosion of construction materials in flash chambers and 

water boxes has been studied adequately by a number of investigators. However, the 

major problem in both acid and additive MSF plants is vapor-side corrosion. While in 

acid dosed plants, almost all the CO 2 is removed by the decarbonator, CO 2 is present 

along with O 2 as non-condensable gases in additive treatment plants. Other gases like 

H 2 S, NH 3 , Br 2 and Cl 2 may also present in significant concentrations and could be 

responsible for corrosion attack. 

Oldfield and Todd [1987] studied vapor side corrosion in an MSF plant. It was 

concluded that in acid dosed plants and probably also in additive plants the vapor-side 

corrosion is caused primarily by the presence of oxygen, this was coming not from the 

deaerated brine but from air-in-leakage into the evaporator. The results of studies on the 

corrosion behavior of stainless steels were reported [Saricemen et. al. 1990]. In air 

ejector system of an MSF plant using stainless steel 316L and 304 L steel pipes, while 

316L failed by SCC, the latter grade appeared to be quite resistant to corrosion. Failure 

of SS 316L steel was attributed to the presence of Br 2 in vent gases [Oldfield et al. 

1981]. Lee, Oldfield and Todd [1983] reported failure of 316L components in the 

venting system of MSF in Qatar due to bromine. The presence of ammonia in seawater 

above the normal background level is necessary for this to occur. After 25 years of 

operation, Abu Dhabi desalination plant experienced corrosion problems in its 23 large 

distillation units [Al Sum et. al. 1994]. The deterioration of the protective oxide film 

appeared to he the main cause of the problem which could be solved by improving the 

efficiency of the deaerator and the venting system. In other case [Mc Gregor et. al. 

1995], a group of 24 MSF distillers experienced failure of tubes in the first few high 

temperature stages after 50,000 to 100,000 hours of operation. It was concluded that the 

problem of tube failure was due to the presence of pockets of CO 2 gas near the tubes, 

which redissolves in distillate to form an aggressive solution. The installation of an 

auxiliary vent to reduce the size of the gas pocket allowed a significant increase in 

distillate pH and reduction in dissolved copper. 

The selection of corrosion monitoring system for plant is considered on the basis of 

parameters controlling the performance of the plant. Environment (corroding or non- 

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corroding or both), materials of construction and design are the important factors to be 

considered. A wide variety of methods or techniques are available for carrying out 

corrosion monitoring. The choice of a particular corrosion monitoring technique 

depends upon the type of information required during monitoring which include : defect 

parameter to be measured, access to the stream, equipment reliability and response. 

Electrochemical, analytical and nondestructive (ND) testings are the methodologies 

used for corrosion monitoring. The different techniques available for corrosion 

monitoring include : linear polarization resistance (LPR) [Fontana 1986, Ijsseling 

1986], electrical resistance [Schollen 1976], corrosion coupon testing, analytical 

radiography [Berger 1989], ultrasonic, eddy current, acoustic emission [Pollock 1984], 

infra red imaging, remote visual testing (RVT), thin layer activation (TLA) [Asher et. 

al. 1987] and field signature method [Strommen et. al. 1993]. Due to limitations on the 

amount of information which one can obtain from any single technique, simultaneous 

use of more than one technique is always preferable in order to obtain meaningful data 

on corrosion process. 

Corrosion monitoring in SWCC seawater desalination and power plants is an important 

project placed in the first priority list of the SWCC R&D tasks. As a first phase of the 

project, the corrosion monitoring of Al-Khafji Phase II desalination was undertaken in 

January 1992. A technical report SWCC (RDC) - 34 was submitted in July 1994 

containing the results of 18 months monitoring studies carried out in the flash chambers 

(in the brine flashing zone) and water boxes in desal unit # 100 and # 200. In November 

1995, the progress of the project was internally reviewed and it was decided to complete 

the studies by the end of 1996. Whilst the corrosion monitoring studies carried out up to 

1995 were almost entirely concentrated on monitoring in flashing brine, the work 

carried out during the year 1996 was entirely focused on Corrosion Monitoring in vapor 

phase. This report presents the results of vapor side corrosion monitoring of evaporator 

and vent line in unit # 100 of Al-Khafji desalination plant phase II. 

In the following section, a summary of the results of corrosion monitoring of flash 

chambers and water boxes is given, the results are discussed in detail in aforementioned 

report submitted in July 1994. 

Corrosion Monitoring of Flashing Brine Zone 

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The corrosion monitoring of flash chambers (in the brine flashing zone) and water boxes 

in desal unit # 100 and 200 was carried out in lower (4, 6-9), middle (10-12) and last 

(19) stages of evaporator and 5-6, 7-8, 9-10 and 11-12 stages of water box. Weight loss 

and polarization resistance measurements were used to determine corrosion rates. 

Stainless retractable corrosion coupon holders carrying carbon steel or cupronickel 

samples were fixed in flash chambers or water boxes for weight loss measurements. 

Electro-chemical polarization probes fixed with carbon steel or cupronickel electrodes 

were used to measure corrosion rates at regular time intervals. For weight loss 

measurements in flash chambers or water boxes, the exposure time varied from 45 days 

to 130 days. 

Analysis of corrosion data from electrochemical studies and weight loss measurements 

indicated that by and large, overall corrosion rates of carbon steel and cupronickel in 

flashing brine and water box, respectively, were well within allowable range. 

Modification in the present methodology for improving the levels of accuracy and 

reliability and its implications on plant operation are discussed. 

Vapor Phase Corrosion Monitoring of Al-Khafji MSF Desalination Plant 

SWCC Al-Khafji MSF desalination plant was commissioned in 1985. It has a designed 

capacity of producing 22727 m 3 /d (5 MIGD). There are 2 desal units (# 100 and # 200), 

each unit has 22 stages. The material information and construction particulars for the 

evaporator and the water boxes are provided in Table 2. This plant is an additive dosed 

plant and in this plant no chemical scavenging of oxygen of make-up is carried out. 

Vapor-side corrosion monitoring for this project was carried out in unit # 100 of this 

plant. 

2. OBJECTIVES 

Following are the objectives of the project on Corrosion Monitoring in Al-Khafji 

Desalination Plant: 

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(a) To determine the corrosion rates of constructional materials in flashing brine 

at different locations by using weight loss/coupon method. 

(b) To carry out on-line corrosion monitoring of the flash chambers by 

electrochemical method using Liner Polarization Resistance (LPR) probes. 

(c) To determine corrosion rate in vapor phase region of the evaporators and 

vents by fixing coupons and retractable coupons holders. 

The objectives under (a) and (b) had already been achieved and the results of the studies 

were incorporated in the technical report - SWCC (RDC) -34 issued in July 1994. This 

report includes the results of the studies carried out to achieve the above mentioned 

objective (c). 

3. EXPERIMENTAL 

3.1 Materials 

Commercially available carbon steel (0.3% C), SS316L stainless steel and 70/30 Cu-Ni 

alloy were used for corrosion monitoring. 

3.2 Technique 

Weight loss coupon technique was used for corrosion rate measurements following 

ASTM standards for Laboratory Immersion Corrosion Testing of Metals. (G31-72, 

reapproved 1-90) and Conducting Corrosion Coupon Test in Plant Equipment (G4-84). 

Vapor phase corrosion monitoring was done by fixing coupons in the venting line and 

flash chambers. The operational parameters of the plant were closely monitored, and 

any variation in the parameters reported during operation was taken into account. 

Following is the brief description of the technique and procedure involved. 

The weight loss technique is used so as to enable the corrosion rate measurements to be 

done without disturbing the plant operation. It has the advantage that the corrosion 

which has actually occurred can be observed on the sample. Moreover, this technique 

allows a visual examination, physical measurements and the chemical analysis of the 

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corrosion products. Corrosion coupons can be mounted in different configurations to 

study different types of corrosion mechanisms, such as crevice corrosion, galvanic 

attack and stress corrosion. Coupons, which are the test specimens of the material of 

interest, are carefully cleaned, weighed and measured before being assembled on the 

corrosion test retractable probes. During assembly, two coupons are arranged so that 

they are electrically insulated from the probe material so as to avoid galvanic attack. 

After a prescribed period of exposure, the corrosion probe is disassembled and the 

corroded coupons are chemically cleaned, weighed and corrosion rate is determined. 

The corrosion rate is calculated from the weight loss, time of exposure and original 

exposed surface area of the material by the following formula [Fontana, 1987]. 

where : 

MPY = 

Mils Per Year 

W = Weight loss, mg 

A = Area of specimen, in 2 

MPY = 534 W 

AtD 

t = Exposure time, hr 

D = Density of specimen, g/cm 3 

In brine immersion zone, the coupon test method could be used successfully by using 

large surface area coupons. The most serious problem encountered in immersion zone 

is the high probability of scale formation. The scales can be adherent to metal which 

give an extra protection to the metal or could be non-adherent which could lead to 

initiate under-deposit corrosion. The difficult problem in calculation of corrosion rate 

of immersed coupon is the determination of actual surface area due to the scale 

formation and time parameters of this process. 

During coupon testing, the coupon could be mounted inside the flash chamber at 

different locations with different angles. The coupon should be insulated from flash 

chamber materials, fastener and holder materials by using any non-conductive material. 

3.3 Methodology 

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Coupons of carbon steel, AISI 316 L and 70 Cu-30 Ni were fixed above the demisters in 

different stages of evaporator and with the help of the retractable coupon holders in the 

vent lines of different stages. Figure 1 shows the photo of cascading vent line of unit # 

100 at Al-Khafji plants where the coupons were fixed. Coupon locations and exposure 

periods in flash chambers and vent lines and their arrangement is shown schematically 

in Figures 2. and 3 

Coupon Locations 

Exposure Period 

1. Vent line stages # 3, 8, 13, 17 and 22 2000 and 4000 hr. 

2. Evaporator stages # 1, 4, 10, 14 and 18 9000 hr. 

The results of corrosion monitoring in vent line show comparatively high corrosion rates 

in central stage # 13. In order to investigate this behavior in more detail, coupons of 

carbon steel and cupronickel were also fixed in stages # 9, 10, 11 and 12 for 2,000 hrs 

exposure. The arrangement of coupon placement is shown schematically in Fig. 4. 

Coupons fixed in the vapor-side of the evaporator were taken out after 9,000 hrs., while 

coupons fixed in vent lines were taken out after 2,000 and 4,000 hrs of exposure. 

4. RESULTS AND DISCUSSION 

Vapor side corrosion monitoring in vent line and flash chambers of MSF desalination 

plant, Al-Khafji was carried out by exposing the carbon steel, cupronickel and AISI 316 

samples. The choice of 70-30 cuopronickel was mainly due to its ready availability and 

its application in last stages of the evaporator. Moreover, chemical reactivity wise there 

is no significant difference between 90/10 and 70/30 cupronickel. The specimens were 

exposed for 2,000 and 4,000 hr in vent line and 9,000 hrs. above the flashing zone of 

evaporator stages. 

Figures 5 to 8 show photographs of carbon steel, AISI 316L and 70/30 Cu-Ni alloy, 

respectively, exposed to vapor in flash chambers of desalination unit # 100. Heavy 

corrosion is observed in carbon steel specimens. It can be markedly observed that the 

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carbon steel coupons exposed in early stages (# 4) formed black scales which are very 

adherent while in the later stages (# 10 onwards) the scales formed are rust colored and 

loose. After removal of scales, pitting can be seen on the surface of alloys (Fig. 9), the 

pitting is more extensive in later stages. Considerable amount of corrosion products 

were observed on cupronickel alloy exposed in stage # 4 while very little corrosion was 

found on cupronickel alloy coupons exposed in stage # 10 onwards. No or negligible 

corrosion is observed on 316L specimen in any stage of the evaporator. Figure 10 shows 

a photograph of cupronickel alloy specimens exposed to non-condensable gas in the 

vent line for 2,000 hrs, appreciable corrosion is found on the specimens. Fig 11 shows a 

photograph of carbon steel exposed for the same period. Heavy corrosion product 

formation is noticed in all stages. Fig. 12 shows a photograph of SS 316L coupons 

exposed in vent lines at different stages; no or very little corrosion is found. 

Out of the two coupons of each alloy exposed in the vapor zone of evaporator, one 

coupon was used for EDAX and metallogaphy and another for weight loss and corrosion 

rate measurement. Considering the corrosion behavior of alloys in vapor zone of 

evaporator and vent line (Fig. 13 and 14) the following trends are noted: 

Evaporator Satges 

(i) In early stages (up to # 4) corrosion of cupronickel is more severe than latter 

stages. 

(ii) Carbon steel shows much lower corrosion in early stages (up to # 4) as 

compared to higher stages. 

(iii) Corrosion of SS 316L insignificant and almost the same in all the stages. 

In vent line 

(i) 

Weight loss in the coupons of different alloys are in the following 

increasing order: 

SS 316 L > Cu-Ni 70/30 >> Carbon Steel 

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(ii) In case of carbon steel and cupronickel alloy, corrosion in early stage (#3) is 

much higher as compared to the later stages. 

(iii) Corrosion of SS 316 L is very small and almost same in all the stages. 

(iv) In middle stage (#13) corrosion of all the three alloys is higher as compared 

to stage # 8, 17 and 22. 

In vent lines (Figs.14 to 16) weight loss decreases steeply from stage # 3 reaching a 

minimum at stage # 8 and then increases showing a maximum at # 13. A maxima in 

corrosion rates has been generally found in the middle stages # 10-13 in all the alloys. 

This could be explained on the basis of temperature, cascading of non-condensable 

gases and air in-leakage providing most favorable conditions for corrosion at these 

stages. 

Figure 17A shows that the weight losses of SS 316L specimens are much higher (about 

one magnitude) in vent lines than that in flash chambers although the magnitude of the 

weight loss is quite low. In case of vapor side corrosion of cupronickel exposed in vent 

line and flash chambers, in middle stages (8-17), weight losses after 4,000 hrs. in vent 

line are more or less same as in flash chambers after 9000 hr, however, in early stage(up 

to # 8 in vent line Cu-Ni alloy corrodes much faster as compared to the flash chambers 

(Fig 17.B). 

Figures 18A to 22A show some typical scanning electron micrographs of the crosssection 

of mild steel coupons exposed for 9,000 hr in different stages of evaporator. 

All the microstructures have a common feature that the scales are copious and adherent. 

The inner scales are generally separated from alloy interface. The energy profiles 

obtained by EDAX for the exposed alloy indicate the presence of Fe and O in the outer 

layers and that of chloride in the inner layers in all stages (Fig. 18B and 22B). In some 

cases presence of Al, Si and S is also indicated. Chloride is invariably concentrated at 

the alloy/scale interface. Cracking of otherwise compact and adhered scales on mild 

steel exposed to non-condensable gases in the evaporator could be attributed to the 

presence of chloride at the metal/matrix/ oxide scale interface. 

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Figures 23 to 27 show some typical energy profiles of the mild steel samples exposed to 

non-condensed gases in vent lines. These energy profiles show formation of iron oxides 

as the main corrosion product with some chloride. It is important to notice that sulfur is 

not present in considerable amount as it has been found in the corrosion products of Cu- 

Ni 70/30 alloy coupons (Fig.31-34). It is also noteworthy that small amount of Cu is 

present in the corrosion product. 

Table-3 summarizes corrosion rates of coupons of all three alloys exposed in the vent 

lines at different stages Corrosion rates of carbon steel are much higher than SS 316L 

and cupronickel alloy. In stage # 3 corrosion rates of cupronickel are about one 

magnitude lower to carbon steel. The corrosion rates of SS316L in vent line are 

extremely low at all the stages and are at least one order of magnitude lower than 

cupronickel alloy. 

Table-4 gives corrosion rate values of all the three alloys exposed to non-condensable 

gases in the evaporator. These corrosion rates are much below the allowable limit. The 

corrosion rates for carbon steel are higher in stage#10 onwards than those observed in 

the initial four stages. Corrosion rates of cupronickel are highest in stage # 1 (0.1 MPY) 

and lowest in stage # 18 (0.007 MPY). The lowest corrosion rate could be explained by 

the accumulation of non-condensable gases which is at its minimum in stage #8. 

Corrosion rates of SS 316L in all stages are extremely low (0.001 to 0.002 MPY) 

showing virtually no corrosion in evaporator by non-condensable gases. 

It appears that a decrease or increase in corrosion rates is directly related to build up of 

non-condensable gases in vents and subsequent cascading. As the non-condensable 

gases concentration increases corrosion rates are also increased. With cascading, 

corrosion rate goes down but increases again from stage to stage as non-condensable 

gases concentration builds up. In early stages, corrosion rates are high due to elevated 

temperatures and high concentration of corrosive gases in the vapor phase. 

Comparing the corrosion behavior of mild steel in flash chambers and vent lines, higher 

corrosion of carbon steel in early stages (1-3) is observed in vent line as compared to 

flash chamber (Tables 3-4). In stages # 8 and onwards, corrosion rates are higher in 

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flash chamber than in vent line. In evaporators of stages 8 onwards, the vapor phase is 

mainly consisted of moisture and non-condensable corrosive gases which provide 

favorable conditions for rust formation and could be the reason for high corrosion rates. 

In vent lines, mild steel is attacked significantly by non-condensable gases only in early 

stages perhaps due to high temperatures. 

Considering, vapor-side corrosion of cupronickel 70/30 exposed in vent line and flash 

chambers, the corrosion rates in vent line are much higher than in flash chambers. In 

vent lines, the cupronickel alloy is attacked much more severely due to the presence of 

predominantly corrosive gases. In flash chambers, there is predominant concentration of 

water vapor and relatively low concentrations of non-condensable gases and in 

consequence, attack is not so severe. The corrosion rates of SS 316L in vents are 

extremely low (0.002 to 0.07 MPY) but still one order of magnitude higher than flash 

chambers (0.001 to 0.002 MPY). In either case, the corrosion rates are not alarmingly 

significant and the 316 L can be safely used in vent lines. 

Figures 28A to30A show photomicrographs of the cross section of cupronickel alloy 

exposed to non-condensable gases in flash chambers of different stages. The scales are 

porous and show disruption of scale layers although inner scale layer is partially intact. 

EDAX profiles (28B to 30B) of the scales show the presence of S and Cl with Si and Al 

in smaller amounts. The morphology of the scales points out the presence of copper and 

nickel sulfides which normally form inadherent and fragile scales. The EDAX profiles 

(Fig. 31 to 34) of the corrosion products obtained from cupronickel specimens exposed 

in vent line indicate the presence of high concentration of sulfur and some iron, chloride 

is virtually absent. There are two major differences in the composition of the scales : 

the scales on cupronickel specimens exposed in flash chambers contain largely chloride 

and sulfur whereas scales from vent line specimens contain high concentration of sulfur 

and iron but no chloride. The sulfur appears to be in the form of copper and nickel 

sulfides. 

An interesting feature of the corrosion products from the cupronickel alloy specimens 

exposed to non-condensable gases in flash chambers or vent lines is that sulfur is 

invariably present in significant concentrations. The concentration of sulfur in vent line 

corrosion products is much higher than the corrosion products from flash chambers. 

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This provides a plausible explanation for the significantly higher corrosion rates of 

cupronickel alloy in vent line. The corrosion products from carbon steel specimens 

exposed to non-condensable gases in flash chambers invariably contain sulfur though in 

small concentrations. The vent line corrosion products contain no or insignificantly low 

concentration of sulfur. Copper is present in small but in perceptible concentrations in 

corrosion products of steel exposed in vent lines. There is no plausible explanation for 

the existence of copper in the scales or corrosion products of the carbon steel coupons. 

A detailed systematic study in collaboration with chemistry and Biology Departments 

shall be useful in investigating the source of sulfur in the non-condensable gases. The 

EDAX data also indicate the presence of significant concentration of bromide in the 

corrosion products. The source of bromide could be the seawater. 

Considering the corrosion behavior of materials in vent line and evaporator at different 

stages, the shape of the weight loss vs. no stages of curves can be explained broadly as 

follows: 

(i) 

(ii) 

Steep increase in weight loss (or corrosion rates) is due to breaking up or 

spalling of scales when fresh alloy surface comes into contact with gases. 

Decrease in weight loss could be due to progressive formation of adherent 

protective scales. 

(iii) Presence of plateau (no change in weight) could be due to the formation of 

protective scales. 

An analysis of the corrosion rates data (by weight loss technique) from corrosion 

monitoring studies in flashing brine in the evaporator and water boxes provide some 

useful information regarding the corrosion of carbon steel and cupronickel alloys. 

Whilst the corrosion rate of carbon steel varies from 0.6 MPY (stage # 6) to 1.8 (stage # 

19) after 25000 hours, the corrosion rates of cupronickel (70-30) in water boxes are 0.18 

(# 7-8, unit 100) and 0.13 (# 5-6, unit 200) after the same exposure time. The low 

corrosion rate values for both carbon steel and cupronickel (70-30) alloys are well 

within the allowable range. Comparing these values with those for vapor phase 

corrosion in evaporator (9000 hrs exposures) and vent line (2000 and 4000 hrs 

exposures), corrosion rates are of the same magnitude. 

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Cases of failure of desal condenser tubes in some MSF desalination plants in Middle 

East are referred in the introduction section of this report. The main cause of the failure 

has been attributed to the attack of non-condensable gases present in flash chamber 

and/or vent line. R&D Center also investigated some cases concerning with the failure 

of desal tubes in SWCC plants. During corrosion monitoring in Al-Khafji plant, a lot of 

corrosion was found on the carbon steel wall above the heat transfer tubes (Fig. 35) and 

70-30 cupronickel alloy heat transfer tubes, unit # 100 (Fig. 36) which appears to be he 

result of vapor phase attack. In Jeddah Phase IV, the failure of desal tubes in stage #4 

has been attributed to dropping of acidic condensate from closed vent pipe which 

resulted in corrosion of tubes. In Al-Wajh desalination plant run on multi effect 

evaporation, the erosion-corrosion failure of desal tubes is caused due to the reaction of 

metal and the non-condensable gases or vapor pockets present at the bottom section of 

evaporation chamber. In both the cases, the R&D Center found the inadequacy of the 

venting system as the main cause of the problem. Improvement in the venting system 

can prevent or at least minimize the failure of desal tubes by vapor phase corrosion 

attack. 

5. CONCLUSIONS 

The following conclusions can be drawn from the vapor phase corrosion monitoring 

studies in Al-Khafji desalination plant: 

(i) Corrosion rates of cupronickel and stainless steel 316L exposed to noncondensable 

gases in vent lines are much higher than those exposed in 

different flash chambers. 

(ii) In initial stages, corrosion rates of carbon steel exposed to non-condensable 

gases in vent lines are higher than those exposed in flash chambers but at 

higher stages the rates in flash chambers become much higher. Higher 

corrosion rates are attributed to the severe rusting of carbon steel in presence 

of moisture and oxygen. 

(iii) Stainless steels 316L in general show very little or no corrosion either in 

vent lines or flash chambers. 

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(iv) Sulfur is invariably present in the corrosion products of Cu-Ni exposed in 

vent line. High corrosion rates of cupronickel in vent line are attributed to 

the presence of sulfur in non-condensable gases. 

(v) A maxima in corrosion rates has been generally found in the middle stages # 

10-13 in all the alloys. This could be explained on the basis of combined 

effect of temperature, air in-leakage and cascading of non-condensable gases 

providing most favorable conditions for corrosion at these stages. 

(vi) Corrosion rates of all the alloys used appear to be within allowable limits. 

6. RECOMMENDATIONS 

A considerable number of cases related to corrosion of heat transfer tubes in some Gulf 

Desalination Plants have been reported in recent years and major cause of the failure 

appears to be the vapor phase corrosion attack. Some recent cases of failure of 

evaporator tubes in SWCC plants also point out the key role of non-condensable in 

corrosion attack. The results of corrosion monitoring at Al-Khafji plant provide 

conclusive evidence regarding the involvement of vapors in corrosion of heat transfer 

tubes. On the basis of the present investigation, we recommend the following: 

(i) A survey of all the SWCC MSF plant should be carried out with respect to 

the condition of evaporator tubes. 

(ii) Short term and long term corrosion monitoring of the plant should be 

undertaken using different types of constructional alloys. 

(iii) Preventive measures have to be taken on the basis of the results of corrosion 

monitoring studies. 

(iv) SWCC should make it obligatory to have built in facilities for corrosion 

monitoring in flash chambers, vent lines and other section of the new MSF 

plants. 

(v) As the cause of vapor side corrosion of heat transfer tubes has been attributed 

to the accumulation of non-condensable gases in the venting system, the 

possibility of modification of design of venting system should be seriously 

considered in the existing or the future thermal desalination plants. 

1634

REFERENCES 

1. Al-Sum, E. A., et. al., (1994) "Vapor Side Corrosion of Copper Based Condenser Tubes 

of the MSF Desalination Plants of Abu Dhabi" Desalination, Vol. 97, pp. 109-19. 

2. Asher, J. T. W. Conlon, C. Westcott, (1987) "Thin Layer Activation of Corrosion 

Monitoring and Detection of Pitting" Corrosion, 87, NACE, Paper No. 264. 

3. Berger, H. (1984) Neutron Radiographic : "Detection of Corrosion in Corrosion 

Monitoring in Industrial Plants" Moran and Labine, Editors, ASTM, STP-908 pp. 5-16. 

4. Cigna, R., L. Guiliani, G. Gusmani, (1985) "Continuous Corrosion Monitoring in 

Desalination Plants" Desalination Vol. 55, p. 219. 

5. Fontana, Mars G., (1987) "Corrosion Engineering" 3rd Ed. Singapore. McGraw Hill p. 

14. 

6. Hodgkiess, T., W. T. Hanbury and M. H. Hejazian, (1983) "Corrosion Tests of Range of 

Materials in Three MSF Plants" Desalination, Vol. 4, p. 223. 

7. Ijsselig, F. P., (1986) "British Corrosion Journal" Vol. 21, p. 376. 

8. Lee, W. S. W., J. W. Oldfield, and B. Todd, (1983) "Corrosion Problems Caused by 

bromine Formation in Additive Dosed MSF Desalination Plants" Desalination, Vol. 44, 

pp. 209-221. 

9. McGregor, I. D and S. Karim, (1995) "Tube Corrosion in High Stages of MSF 

Distillers" Proc. IDA World Congress, Abu Dhabi, Nov. 18-24, pp. 307-310. 

10. Narain, S. and H. Sami, Asad, (1992) "Corrosion Problems in Low-Temperature 

Desalination Units", Materials Performance, Vol. 31(4), p. 64. 

11. Nordin, S., (1983) "Studies on Stainless Steels for Services in Desalination Plants" 

Desalination, Vol. 4, p. 255. 

12. Oldfield, J. W. and B. Todd, (1979) "Corrosion Considerations in Selecting Metals for 

Flash Chambers" Desalination, Vol. 31, p. 365. 

13. Oldfield, J. W. and B. Todd (1981) "Corrosion Problems Caused by Bromine Formation 

in MSF Desalination Plants" Desalination, Vol. 38, pp. 233-245. 

14. Oldfield, J. W. and B. Todd, (1987) "Vapor Side Corrosion in MSF Plants" Desalination 

Vol. 66, p. 171. 

15. Pollock, A. A., (1984) "Acoustic Emission Detection of Corrosion in Corrosion 

Monitoring in Industrial Plants, Moran and Labine, Editors, AST, STP-908, pp. 5-16. 

1635

16. Saricimen, H., et. al. (1990) "Performance of Austenitic Stainless Steels in MSF 

Desalination Plant Flash Chamber in Arabian Gulf" Desalination Vol. 78, p. 327. 

17. Scholden, D. L., (1976) "Analytical Radiography Corrosion" 76, NACE, Paper No. 40. 

18. Strommen, R. D. H. Horn and K. R. World, (1993) "A Unique Method for Monitoring 

Corrosion of Steel Piping Vessels. Materials Performance, Vol. 32(3), p. 50. 

19. SWCC Annual Report 1994. 

Table 1. 

Corrosion resistance 

in oxygenated 

seawater at low 

velocities 

Corrosion resistance 

in hot deaerated 

moderate velocities 

Stress 

cracking 

Summary of Properties of Carbon Steel, 90/10 Cupronickel and type 

316 Stainless Steel for Evaporator Components 

corrosion 

Carbon steel 90/10 Cu Ni Type 316 SS Remarks 

Moderate, 

about 0.1 

mm/yr. plus 

pitting 

Poor, limited 

low velocities 

and 

temperature 

(see test 

Good < 0.02 

mm/yr. 

Good 

Suffers pitting 

and crevice 

corrosion 

Good if well 

deaerated 

and/or velocity 

is high 

Resistant Resistant Resistant if 

oxygen level is 

low and pH is 

7 or over 

Availability All forms Plate and pipe 

+ clad plate 

All forms 

including clad 

pate 

Approximate cost 1 15 10 

ratio in platform 

Weldability Good Good Good* 

* A low carbon grade is often used to avoid intergranular corrosion in the HAZ, e.g. AISI 316L 

This condition 

might be 

encountered 

during shut 

down if 

drainage is 

incomplete 

- 

Care is needed 

with SS if O 2 

can leak in or 

brine out. 

1636

Table 2. Materials and Construction Information of Evaporator, Al-Khafji Plant 

A. Material Information 

Shell material: 

Carbon steel (All stages) 

Cladding material: AISI 316L (stage 1-3) 

B. Construction Particulars 

Number of units: 2 

Total number of stages: 22 

Recirculation or once through: Recirculation 

Reject stages: 3 

Lined stages: 

1, 2 and 3 (with SS 316L 3 mm) 

Extent of lining: 

Floor + side wall + partition walls + roof 

Bare stages: 19 

Corrosion allowance: 12 mm 

Manhole: 

316 lined carbon steel 

Location: 

Side 

Size: 

500 mm 

Type: 

Circular 

Flashing brine gate: 

Adjustable 

Distillate gates or ducts: Fixed 

C. Production (MIGD): 2.5 (each unit) 

D. TBT ( o C): 90-112 

E. Tube bundles 

Heat rejection: 

heat recovery 

Brine heater: 

Brine heater shell: 

Ti (3 stages) 

90/10 Cu-Ni (19 stages) 

66/30/2/2 Cu-Ni-Fe-Mn 

Carbon steel 

1637

Table 3. 

Corrosion of Cu-Ni, Carbon Steel and Stainless Steel 316L in 

Vent Lines of MSF Desalination Plant, Unit # 100, Al-Khafji 

S. No. Distiller Stage # 

(Cascading) 

Exposure Period 

(hr.) 

Corrosion Rate (MPY) of 

Alloys 

70-30 

Cu-Ni 

SS 

316L 

Carbon 

Steel 

1 3 2000 0.41 0.06 3.97 

(1-3) 4000 0.42 0.05 5.55 

2 8 2000 0.07 0.06 0.46 

(4-8) 4000 0.05 0.04 1.02 

3 13 2000 0.1 0.07 1.95 

(9-13) 4000 0.06 0.07 1.74 

4 17 2000 0.04 0.06 0.42 

(14-17) 4000 0.01 0.02 0.99 

5 22 2000 0.08 0.04 0.87 

(18-22) 4000 0.12 0.02 0.96 

Table 4. 

Corrosion of Cu-Ni, Stainless Steel 316 L and Carbon 

Steel in the Flash chambers of MSF Desalination Plant, 

Unit # 100, Al-Khafji 

S. 

No. 

Distiller 

Stage # 

Exposure 

Period (h) 

Corrosion Rate (MPY) of 

Aloys 

70-30 

Cu-Ni 

SS 316 L 

Carbon 

Steel 

1 1 9000 0.1 0.001 1.80 

2 4 9000 0.05 0.001 1.72 

3 10 9000 0.02 0.002 2.41 

4 14 9000 0.01 0.002 2.40 

5 18 9000 0.007 0.001 2.36 

1638

.-. 

,. - . . . . 

Fig. 1 

Photograph showing cascading and retractable coupon 

holder fixed in the vent line 

1639

1640

1641

1642

Fi 

SS3 16L exposed in the vapor side of flash chamber #4 of MSF plant for 

9000 hr. 

Fig. 6 

Photograph showing coupons of (left to right) Cu-Ni alloy, Carbon steel and 

SS3 16L exposed in the vapor side of flash chamber #lO of MSF plant for 

9000 hr. 

1643

Fig. 7 



9000 hr. 

Fig. 8 



9000 hr. 

1644

Stage-8 Stage-13 Stage-l 7 Stage-22 

FiQ. 9 Pitting condition of the coupons of Carbon steel exposed for 9000 

hr. in different stages of evaporator of MSF plant. Note the 

increase in pitting with the stages. 

Fig. 10 

EVAPORATER # 3 8 13 16 22 

Cupro-Nickel 70-30 alloy specimens exposed to vent lines of different 

stages of MSF plant for 2000 hr. 

I 

1645

Fig. 11 

Carbon steel specimens exposed to vent line of different stages of 

MSF plant for 2000 hr. 

Fig. 12 

Stainless steel specimens exposed to vent line of different stages of 

MSF plant for 2000 hr. 

1646

1647

1648

1649

1650

1651

1652

1653

1654

1655

1656

1657

1658

1659

500- 

[b] 

O- 

0 

Energy (keV) 

Fig.30 

[a] SEM Picture [b] EDAX spectrum of cross marked area of oxide 

scale formed on the Cu-Ni (70-30) coupons exposed to the vapor side 

in the flash chamber #l0 of MSF plant. 

1660

1661

1662

Fig. 

Ivery 

Fig. 36 

General view of carbon steel roof above the heat transfer tubes in the heat 

recovery section of Al-Khafji MSF plant. 

1663

CORROSION MONITORING OF SWCC PLANTS - I:1 Vapor-Side ...

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