Paper 032 Durability Guidelines for Materials in SWRO Brine_final

7/26/2019 Paper 032 Durability Guidelines for Materials in SWRO Brine_final

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Corrosion & Prevention 2010 Paper 032 Page 1

DURABILITY GUIDELINES FOR MATERIALS IN

AGGRESSIVE BRINE EXPOSURES

F. Blin1& S. Furman

1

1AECOM

The authors were awarded the AC Kennett Award for best paper deali ng with non-

metall ic corr osion at the 2011 Corrosion & Prevention Conference.

SUMMARY:

Extended drought conditions and an increasing population have lead to water supply uncertainty in

Australia. In recent years major cities in Australia have supplemented or are planning to supplement the

water supply with desalinated water produced using reverse osmosis technology.

The by-product of this process is concentrated, ambient temperature brine. This aggressive fluid posessignificant durability issues for many concrete and metallic materials that are required to transport it back

to the ocean. If not addressed adequately such durability issues could significantly impact on the

operation of a desalination plant. However, existing standards do not provide sufficient guidance on how

to select materials that withstand exposure to such aggressive brine solutions.

This paper provides an understanding of the degradation mechanisms that may impact on the materials

used in the brine circuit of a seawater reverse osmosis desalination plant. Based on theory and

experience, it also sets out an approach for the selection of concrete and corrosion resistant alloys for use

in the transportation and dispersion of brine.

Keywords:Durability, Corrosion, Reinforced Concrete, Metals, Brine Exposure.

1. INTRODUCTION

Since first appearing in the 1970s in the Middle East, desalination plants are now found in more than 150 countries around the

world. This is due to a combination of population growth, increase of industry and agriculture, and fresh water scarcity [1].

The method of producing fresh water from the sea or brackish groundwater has been evolving from a distillation process to

Reverse Osmosis (RO) process, where water is forced at high pressure through a membrane that separates salts from the water

[1]. New techniques being researched to reduce the energy required by the desalination process include forward osmosis,

carbon nanotubes and biometics [1]. With each change in technology there are associated durability challenges forconstruction materials.

This paper considers the guidelines provided in ISO 13823 - general principles on the design of structures for durability [2].

This standard, first published in 2008, aims to improve the evaluation and design of structures for durability by theincorporation of building-science principles into structural-engineering practice [2]. As such, it uses a similar terminology

and limit-state approach for structural engineering as noted in standards such as ISO 2394 [3].

2. ENVIRONMENTAL EXPOSURE: BRINE

Brine is the reject stream from the Seawater Reverse Osmosis (SWRO) process which contains concentrated levels of the ionic

species present in the feed stream. SWRO desalination plants commonly use a two-stage RO system with the most

concentrated brine produced from the first stage. This stream is effectively highly concentrated seawater with a resultant

increase in aggressivity to materials compared with seawater. Further references to brine within this paper specifically mean

this first stage reject concentrated stream.

Before discharge to the ocean outfall, the brine may be used in pressure exchange energy recovery devices and backwashing

the seawater pre-treatment filtration system [4]. This backwashing circuit, which includes a clarification system, together with


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the first pass RO discharge, the energy recovery circuit, and ocean discharge system, are effectively the brine circuit.

Pumps, valves, storage tanks, clarifiers, outfall tunnels, and diffuser systems are some of the equipment and structures that

comprise the brine circuit. The materials for all these components need to be resistant to the effects of brine or be maintainable

for the nominated design life.

SWRO desalination plants operating in Australia prior to 2008 typically produce brine with seawater ion concentrations of 1.5

to 1.8 times that of seawater [5]. This concentration factor is gradually increasing as membrane and processing technologies

improve, and modern plants are approaching a brine concentration of 1.9 times that of seawater. For the purpose of this paper,

a 1.9 concentration factor is considered, that is, a brine solution with a chloride concentration up to 39,500 ppm and a sulphateconcentration of up to 5,400 ppm.

Table 1: Typical composition of seawater and brine (approximate values)

Environment pH Chlorides (ppm) Sulphate (ppm) Magnesium (ppm)

Seawater 7.58.5 19,30020,900 2,950-3,050 1,300-1,450

Brine 6.57.5 39,500 5,400 2,500

According to ISO 13823, in designing for durability, the structure environment (the macro-environment) contains influences

outside the structure (atmospheric and ground conditions, including pollution) and inside the structure (indoor atmosphere andmaterials), that are transformed into one or more agents on the surface of or within a component (the micro-environment)

causing environmental action. In the case of SWRO brine, the influences (structure environment) would be defined as outsideand inside water (that is, the fluid) and the agents causing environmental action are chlorides, sulphates and magnesium as

listed in the table above. However, this environmental exposure is not easily classified using the key Australian Standards for

concrete and steel structures, as illustrated in the following table.

Table 2: Environmental classifications to Australian Standard

AS 5100.5 [6] AS 3600 [7] AS 4997 [8] AS 2159 [9] AS 3735 [10]2

Design life (yrs)1 100 50 20% 50 50 & 100 4060

Concrete exposure

classification

U U Not defined Not defined B2C

Steel N/A N/A Not defined Not defined N/ANotes:

1. The design lives specified in this table are as defined in the respective Australian Standards.

2. Guidance is provided in Supplement 1.

3. The classification depends on whether elements are predominantly submerged or in alternate wet and dry conditions.

As shown above, while the exposure classifications for concrete elements exposed to brine can be defined for structures with a

40 to 60 year design life in accordance with AS 3735, no specific guidance is given when a longer life is required. In addition,

none of the standards listed above propose an exposure classification for steel elements in contact with brine.

3. REINFORCED CONCRETE IN CONTACT WITH BRINE

Typically, concrete elements in contact with brine produced in desalination plants are reinforced either using steel bars or steel

fibres. While unreinforced concrete may be used, the cement matrix is still susceptible to attack by sulphate and magnesium

ions as described below.

3.1 Deterioration mechanisms

Within brine solutions, the key agents causing deterioration of the concrete matrix or steel reinforcement are sulphate,

magnesium and chloride ions [11, 12]. The environmental action and its effects are summarised in the following sections

using the terminology in ISO 13823.

Sulphate attack:As for soils, the reaction that causes the expansion and deterioration of the cement matrix depends

on the type of sulphate compound and the constituents of the concrete [13]. The mechanism of sulphate attack is


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described in references quoted in this paper [11, 14-16] but various Australian Standards [6, 7, 9] state that more than

1,000 ppm of sulphate is considered aggressive towards concrete. Magnesium sulphate has been reported to be

potentially very aggressive to concrete [17] though its full impact has been debated [13]. AS 2159 states that

sulphate ions become aggressive at levels of 600 to 1,000 ppm when combined with magnesium or ammonium ions

[18].

Chloride attack:With a chloride concentration nominally 1.9 times that of seawater, the risk of chloride attack of the

steel reinforcement in brine solutions is relatively high. The mechanism of corrosion of steel reinforcement due to

chloride attack, resulting not only in section loss but also in cracking, delamination and eventually spalling of theconcrete, has been extensively published [19]. While chlorides can also react with the concrete paste [11] it has been

reported to potentially moderate the effects of sulphate attack [10, 20]. AS 2159 notes: in the presence of chloride

ions, attack by sulphate ions generally exhibits little disruptive expansion with the exception of conditions of extreme

wetting and extreme drying where crystallisation can cause surface fretting of concrete [9].

3.2 Durability performance requirements

The following sections discuss durability measures to mitigate the risk of deterioration of reinforced concrete elements (using

steel bars or fibres) exposed to SWRO brine based on information obtained from literature as well as experience. The

following discussion is based on the assumption that the fundamental parameters listed below are adequately addressed:

The aggregates that form part of the concrete matrix satisfy geometrical requirements (for example, shape, grading

and size) and have adequate physical properties (for example, porosity and water absorption) and strength [19]. The

aggregates must also comply with the durability requirements of AS 2758.1 [21], in particular to minimise the risk ofreactions with the alkalis present in the cement paste.

The mixing water complies with the requirements of AS 1379 [22], specifically its impurity levels.

Cracking is minimised as it can provide a direct route for contaminants to enter the concrete. Appropriate design,

using standards and tools such as CIRIA C660 [23], as well as adequate joints, concrete specification, curing, and

constructions practices can ensure that the risk or extent of thermal and drying shrinkage cracking or plastic shrinkage

and plastic settlement cracking is minimised. While the literature suggests chloride ingress and corrosion rate are notdirectly linked to crack width [16, 19, 23, 24], this parameter is often used as it can be practically measured and

therefore determine whether a crack requires treatment. Values between 0.10.3 mm have been quoted [19], thelower end of the range being deemed an appropriate limit for concrete exposed to aggressive SWRO brine.

Adequate curing is required to avoid detrimental effects on short- and long-term strength, shrinkage, porosity,

resistance to the penetration of contaminants, resistivity, and surface properties including strength, hardness, and

abrasion resistance. AS 4997 requires All maritime concrete structures should be water-cured for at least 7 days and

preferably 14 days under ambient conditions. (). If forms are stripped within 7 days, then supplementary watercuring should take place to 7 days (Clause 6.3.3(h)). This does not appear to align fully with section 4.5 of AS

5100.5 which states members subject to exposure classification A, B1, B2 or C shall be initially cured continuouslyfor at least 7 days under ambient conditions, or cured by accelerated methods so that the average compressive strength

of the concrete at the completion of the accelerated curing is not less than the appropriate value given in Table 4.5. A

similar clause can be found in AS 3735 (Section 5.2.2). However, the linking of curing efficacy to compressive

strength results is not considered appropriate by the authors. Indeed, the outlet tunnel and diffuser of a SWRO plant

being arguably marine structures, and the exposure classification for brine in AS 5100.5 being U, the recommendation

of 14-day wet curing of AS 4997 would be deemed applicable for concrete elements in contact with this aggressivesolution irrespective of concrete strength.

3.2.1 Plain reinforced concrete

3.2.1.1 Mitigation of sulphate (including magnesium) attack

There are many publications that provide recommendations to design concrete to minimise the risk of sulphate attack. For

instance, ACI-201.2R-01 Guide to Durable Concrete [25] prescribes a low water/cementitious ratio (less than 0.4 for the most

severe exposure) and the use of Supplementary Cementitious Materials (SCMs) in the following proportions by mass: Fly Ash

(FA) content of 25-35%, Silica Fume (SF) content of 7-15%, or Blast Furnace Slag (BFS) content of 40-70% [25]. It also

indicates that using GP concrete with low C3A alone may have adverse effect on the resistance to chloride penetration [25];

therefore Type SR cement is not recommended for exposure to brine solutions.

There does not appear to be a universally agreed approach in the literature to help predict or estimate the likely extent of

sulphate attack, in particular, magnesium sulphate attack; and the relevance of findings from laboratory testing on mortar bars

is still being debated [18]. In the absence of an accepted rate of sulphate deterioration, a cautious approach would be to

allow for a sacrificial layer that could be fully degraded over a concrete elements design life without affecting its structural

integrity. An arbitrary allowance of 3040 mm over 100 years for high-quality concrete using the following recommendations


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has been mentioned in the context of SWRO desalination plants but, to the authors knowledge, this does not seem to be

supported by empirical evidence or documented case studies and may be overly conservative. In the absence of an apparent

synergistic reaction between chloride and sulphate ions, any allowance for sulphate attack in adequately design concrete would

be expected to be less and therefore included within (i.e. not in addition to) the depth of concrete cover over reinforcing bars

required for protection against chloride attack.

3.2.1.2 Mitigation of chloride attack

Standards approach

As mentioned, the SWRO brine environment is not specifically covered by Australian Standards, especially if a long design

life (for example, 100 years) for concrete structures is required. One approach could involve increasing the exposure

classification from C (for alternate wet and dry) to D in AS 3735. While this would equate to a cover of 55 mm, the standard

would actually require isolation from attacking the environment, which may not be the most technical, practical or cost

effective option. Another approach could be to take the maximum cover value published in AS 5100.5, i.e. 70 mm (-5, +10 mmfor formed slabs, beams, walls and columns), but is it appropriate to adopt durability requirements for a C exposure when this

standard would require the designer to classify the brine exposure as U? This illustrates that an approach consisting of

extrapolating the requirements in standards is debatable, requires interpretation and can lead to different outcomes.

Alternatively a different approach based on the modelling of the penetration of chlorides through concrete over time can be

used to estimate the required cover.

Modelling approach

There are many models available in the literature that attempt to predict the time to corrosion initiation and propagation of steel

in concrete. The discussion below is an illustration of a relatively simple deterministic model for corrosion initiation using a

solution to Cranks solution of Ficks Second Law of Diffusion [19]. In the literature, the changes in the diffusion coefficient

over time has been modelled using a maturation coefficient m, also called age factor, which depends on mix proportions

[26,27,28].

Corrosion is considered to have initiated once the chloride concentration at the reinforcement depth reaches a threshold, which

has been reported to vary significantly though a typical value of 0.06% by weight of concrete (which equates approximately to

0.4% by weight of cement for a concrete with a density of 2,450 kg/m3containing 400 kg/m

3of cement) is often quoted for

mild steel [19].

The modelling takes into account the surface chloride concentration, cs, but for exposure in SWRO brine, this is difficult to

estimate. Taking into account the csvalues reported for seawater and the impact of SCMs [19, 27] a csvalue of 1.0% by weight

of concrete could be assumed for elements submerged in brine. For concrete located in a tidal zone in this environment, a

value of 2% could be considered an appropriate upper limit.

Using a model discussed in [28] the relationship between the mix design, its age factor and the chloride diffusion coefficient

measured at 56 days in accordance with NordTest NTBuild 443 is illustrated in the table below. In this example, the depth of

chloride penetration is fixed at 50 mm after 90 years (assuming an arbitrary corrosion propagation to failure period of 10 years

in a 100-year design life scenario).

Table 3: Relationship between mix design and required chloride diffusion coefficient (D56) with a fixed reinforcement cover;

assumed surface chloride and corrosion threshold are 1% (submerged in SWRO brine) and 0.06% by weight of concrete

respectively.

SCM type SCM content (%)Age factor

calculation [29] Age factor Cover (mm) D56(m2/s)

FA 25 0.2 + 0.4(%FA/50) 0.40 60 1.4 10-12

BFS 60 0.2 + 0.4(%BFS/70) 0.54 60 2.6 10-12

The table above illustrates a standard target value for the chloride diffusion coefficient measured by testing is not a val id

approach and instead target values need to be tailored for each concrete mix. This point is further illustrated in the graph below

that shows the difference of chloride penetration depth for two concretes with the same chloride diffusion coefficient measured

at 56 days, but with different age maturation characteristics. In this example, the difference in depth of penetration between the

two concretes is approximately 20 mm over the 90-year period.


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It is not recommended that the covers obtained from the modelling be relied on without the use of safety factors, in a similar

fashion as structural design [19]. In the DuraCrete approach, three values for safety factors are proposed depending on the

criticality of the structure (important or minor, maintainable or inaccessible etc) [29]. This can translate into extra depth of

cover of 20 mm, 14 mm and 8 mm for high, normal and low risks respectively [19]. Given the criticality of the brine circuit for

a SWRO plant, the difficulty to easily access a majority of the structure to undertake inspections and maintenance as well as

the aggressiveness of the medium, a high risk factor would be recommended. Figure 1 shows the cover obtained from themodelling for a typical marine mix (60% BFS, 40% GP) is approximately 52.5 mm, which would be translated to a minimum

design cover of 72.5 mm with the application of a 20 mm extra cover.

Figure 1: Chloride diffusion coefficient vs. depth of penetration over 90 years for two concrete mixes containing SCMs

3.2.1.3 Alternative durability measures

High covers (say in excess of 55 mm) may not always be possible, so alternative options to provide the required durability

may need to be considered. General comments are below:

Stainless steel can be used as reinforcement. Among the various grades available, 316L (UNS31600, austenitic) orGrade 2205 (UNSS31803/S32205, duplex) have been reported to have higher chloride corrosion threshold

concentrations compared to mild steel [19, 30]. It is expected that the threshold is greater than 0.3% by weight of

concrete for these stainless steels [30]. UsingFigure ,it can be shown that at a threshold of 0.3% would suggest acover in the 60% BFS concrete of just under 30 mm to which the 20 mm safety factor can be added for a total cover

of 50 mm. This is a substantially lower cover than if mild steel reinforcement was used.

The installation of cathodic prevention could be investigated as an option based on lifecycle costing and practical

considerations (such as available location for the Transformer Rectifier Unit (TRU), power source etc). As all cabling

TRU and anodes would typically be expected to require replacement approximately every 30 years, the option to use

cathodic prevention for the first 30 years after which the concrete will provide the primary protection on its own (for

example,, the concrete being then required to have a design life of 70 years in a 100-year scenario).

While adequate concrete design and quality control during construction can remove the future need for cathodic

protection (CP) systems for concrete structures, it may still be prudent to provide electrical continuity (via adequate

welding) for the reinforcement located within high risk zones. High risk zones would include any tidal/splash zone in

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

ChlorideContent(%w

tofconcrete)

Cover (mm)

Corrosion Initiation Threshold Mild Steel

S50 25% FA, D56 = 2.0 10-12 m2/s

S50 60% BFS, D56 = 2.0 10-12 m2/s


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contact with brine and/or areas that are difficult to isolate and access for maintenance, or where detailing and, or

construction constraints makes it likely that localised defects may occur.

Protective coatings can be applied to the concrete or the reinforcing bars. While it is preferable to achieve protection

through the design of concrete mixes and cover to reinforcement this is not always possible. Where adequate cover

cannot be achieved, coatings may be required to provide additional protection. For example, precast concrete

building elements that are located where they could be splashed by brine and where concrete cover cannot be easily

increased. In that instance, a silane impregnation treatment could be proposed to provide the required additional

design life to that of the concrete element itself. Proprietary reinforcing steel coatings may have sufficient resistanceagainst chloride attack to provide a large portion, in some instances all, of the design life of the reinforced concrete

element. This is provided that the bond between the coating and the concrete is not negatively impacted. In that case

the mix and cover have to be designed mainly to counter the impact of sulphate attack. While epoxy coating of rebar

is used in the USA and the Middle East and its use is described by ASTM A775 [31] among other internationalstandards, there have been doubts about the long-term durability of this system [19]. Galvanised steel bars can be used

as the passive film formed at the surface of the galvanising is stable in concrete and the corrosion initiation threshold

has been reported to be increased up to 11.5% by mass of cement (approximately 0.160.24% by mass of concretedepending on the cement content) [19]. Although other studies indicate little benefit for galvanising where chloride-

initiated corrosion is the prime attack mechanism.

Linings can become the primary durability measure and there are materials available that have a relatively long design

life in brine solution (see comments in section 5 below). Using a lining can enable concrete covers to be reduced to

say 55 mm as per AS 3735 and also allow the use of concrete with less onerous requirements (with lower

cementitious and/or SCM content, higher water/cementitious ratio etc). The lining effectively acts as a waterproofmembrane within a concrete shell. Underground structures typically require the use of waterproof membranes

between the concrete face and the soil. The lining joints must be carefully designed and treated to avoid water ingress.

Corrosion inhibitors can be added to the concrete mix but while some have been reported to be effective over short to

medium periods of time, there is still some uncertainty regarding their long-term performance. While calcium nitrite

has been extensively used, its anodic nature causes concern due to the risk of increased local corrosion attack in the

case of insufficient inhibitor concentration (for example, when corrosion has initiated and lead to the consumption

of inhibitors reducing concentrations below the required dosage for protection) [32]. Moreover, it is typically not

allowed in reinforced concrete structures permanently immersed in water for environmental and health considerations

(due to leaching of nitrite over time) [32]. Mixed corrosion inhibitors (displaying both anodic and cathodic

behaviour) have been reported to be less efficient but do not carry the risk of accelerated corrosion at low

concentrations [19]. The use of inhibitors has been reported to lead to an increase in the chloride threshold forcorrosion initiation [19] but once again the validity of this approach for structures with relatively long design life (say

more than 40-50 years) is still to be established.

3.2.2 Steel Fibre reinforced concrete

The durability of Steel Fibre Reinforced Concrete (SFRC) is not specifically covered in any Australian Standards. However, it

has been reported to perform well in chloride-containing environments and it does not result in the same pattern of

delamination and spalling as is usually the case for conventionally reinforced concrete structures [33]. However the structuraleffectiveness of the fibres relies on the integrity of their anchorage within the cement matrix. As corrosion will disrupt this, it

would appear valid to apply the same durability principles mentioned above for plain reinforced concrete elements exposed to

SWRO brine to SFRC structures and, in particular, with regards to sulphate (including magnesium sulphate) and chloride

attacks. A key difference compared with conventional reinforced concrete is that chlorides do not have to diffuse down to a

certain depth for corrosion of the steel to initiate. Therefore the following durability design approach is suggested based on an

allowance for sulphate attack and the same modelling tool that predicts the depth of chloride penetration with time:

The mix is designed to minimise the risk of sulphate attack, as outlined above. A sacrificial layer of SFRC is allowed for, in which both the concrete matrix (sulphate attack) and fibres can fully

deteriorate (sulphate attack and chloride induced corrosion).

Beyond this first layer, all fibres can be allowed to fully corrode down to a certain depth (chloride induced corrosion).

The simple modelling tool discussed above could help estimate this maximum permitted depth of chloridepenetration. However, a different model that takes into account a higher corrosion threshold as suggested in the

literature could be developed [34].

This approach is a suggestion only and, while possibly conservative, it has the merit of presenting a scenario that can also be

modelled by structural engineers to check the long-term integrity of the SFRC elements in service.


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4. METALS IN CONTACT WITH BRINE

Within the brine circuit of a SWRO plant, metals and alloys are the materials of choice for regions of high pressure and high

flow rates. Typical components include valves and pumps and the high pressure brine pipes in the energy recovery system.

Guidance on the use of metals in the typical high chloride brine environments found in SWRO plants is very limited. The

preferred source for design engineers is usually Australian or International Standards, although these are usually only pertinent

to steel in natural environments. Information relating to the predicted performance of materials in process environments like

brine is generally not provided in standards.

Alternative sources of information for performance data in brine environments include research and technical articles and

occasionally technical data sheets for specific alloys. Due to the problems associated with the supply of fresh seawater, much

of the early research for high chloride environments was undertaken in sodium chloride solutions. However, as the rate

controlling steps of the corrosion process are often associated with the minor ionic species and the organic materials and

organisms present in seawater [35-36] this research is considered to be of limited use. Other materials research is associated

with distillation desalination processes which produce de-aerated brine [37]. De-aerated brine is less aggressive than the brine

produced through the SWRO process, so this information is also not particularly applicable.

The most commonly used alloys in high chloride environments are currently highly alloyed stainless steels. Although high

nickel alloys and titanium perform well in high chloride brine environments the cost of these materials usually limits their use.


In brine environments containing up to 39,500 ppm chlorides, the most common form of deterioration of metals and alloys iscorrosion. The various mechanisms of corrosion that may be observed include pitting, crevice corrosion, galvanic corrosion,

and erosion corrosion or other flow assisted corrosion mechanisms. The specific corrosion mechanisms that may be observed

are dependent on the type of material, exposure conditions (for example, flow rate) and engineering configuration (for

example, presence of crevices). Overall it is estimated that approximately 40% of all failures in desalination plants are as a

result of pitting corrosion, though many of these may be associated with the higher temperatures encountered in the distillationprocesses used in some desalination plants [38].

4.1.1 Pitting and crevice corrosion

Pitting and crevice corrosion are reported to the most common mechanisms of corrosion of stainless steels in brine

environments. Although there are different ways of assessing stainless steels and nickel alloys with respect to their resistance

to chloride environments, the most commonly accepted measure is the pitting resistance equivalent number (PREN). The

PREN is calculated from the composition of the alloy however due to the different impact of alloying elements in stainless

steel and nickel alloys several different equations are currently used to provide comparative predictive behaviour betweenstainless steel and nickel alloys. The more commonly used equations used to calculate the PREN values are noted below:

For stainless steel alloys PREN=%Cr + 3.3 x %Mo + 16 x %N [39-42]

For Zeron 100 alloy PREN=%Cr + 3.3(%Mo+0.5 x %W) + 16 x %N [39]

For high nickel alloys PREN=%Cr + 1.5(%Mo+ %W+%Nb) [43]

The PREN value for a range of alloys used in desalination plants are presented inTable 4. Experience has found that super

duplex and super austenitic alloy with a PREN greater than 40 usually have adequate pitting resistance to seawater and brine

environments.

Table 4: Chemical composition and PREN values [42]

AlloyUNS

Designation

Nominal Composition (wt %)PRENFe Cr Ni Mo N Other

304L S30403 Balance 18.2 8.2 0 0.06 19

316L S31603 Balance 16.2 10.2 2 0.06 - 24

LDX 2101 S32101 Balance 21.5 1.5 0.3 0.22 - 26

904L N08904 Balance 20 25 4.5 0.06 36

2205 S32205 Balance 22 5.5 3 0.17 - 35

254 SMO S31254 Balance 20 18 6.1 0.20 - 43

2507 S32750 Balance 25 7 4 0.27 - 43

Zeron 100 S32760 Balance 26 8.5 4 0.3 1% Nb 46

AL-6XN N08367 Balance 20.5 24 6.2 0.22 44


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Alloy UNS Nominal Composition (wt %) PREN

Incoloy 825 N08825 22 21.5 42 3 2.25% Cu 26

Inconel 625 N06625 3 22 62 9 3.6% Nb 41

Hastelloy C-276 N10276 6 15.5 5 16 3.9% W 45

PRENs are not the only predictive performance tool for assessing the corrosion performance in chloride environments as charts

have also been developed for assessing the risk of pitting and crevice corrosion. As shown in Figure 2 andFigure 3 thesecharts [44] indicate that the risk of pitting and crevice corrosion for stainless steel increases with temperature and chloride

concentration in the immersed environment. Pitting and crevice corrosion is predicted to occur at chloride concentration

greater than the line drawn for each alloy. However, for 254 SMO (super austenitic stainless steel) as well as high nickel

alloys and titanium, the increase in chloride levels above that of seawater has little impact on the corrosion behaviour of these

materials particularly at the ambient conditions that typically occur in SWRO plants[45]. It is expected that super duplex

stainless steel alloys (e.g. 2507) would behave similarly.

Figure 2: Risk of Pitting Corrosion in chloride environments [44]

Figure 3: Risk of crevice corrosion in chloride environments [44]

Notes: 1. Stainless steel grade EN 1.4307 is a grade 304 stainless steel equivalent to UNS S30403


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2. Stainless steel grade EN 1.4404 is a grade 316 stainless steel equivalent to UNS S31603

4.1.2 Galvanic corrosion

The likelihood of galvanic corrosion in the brine circuit is high due to the complex nature of pumps, valves and other

mechanical equipment that are used. Galvanic corrosion can be controlled by three different strategies:

Electrical isolation of the different alloys.

Only allowing direct connection between alloys within the same category (refer to Table 4).

Ensuring the surface area of the anodic alloy is substantially larger than the cathodic alloy to reduce the rate of

acceleration of corrosion of the anodic alloy.

The risk of galvanic corrosion should be low if the dissimilar alloys both have a similar response to the environment. Table 5below shows the different categories of alloys in seawater. It is possible to connect the materials within each category without

causing galvanic corrosion. Although these categories are for seawater, similar behaviour is expected in brine solutions.

Examples of each type of alloy have been included inTable 5,but this list is not exhaustive.

Table 5: Alloy Groupings for Immersion in Seawater at Ambient Temperature [46]

Category Type Alloy

1 Noble; passive Nickel-chromium-molybdenum alloy (Mo>7%), including;

Inconel 625 (UNS N06625), Hastelloy C276 (UNS N10276) and

Hastelloy C22 (UNS N06022). 6% Mo austenitic stainless steel, including 254 SMO (UNS

S31254), 654 SMO (UNS S32654), Werkstoff 1.4529 (UNS

N08295) Super Duplex Stainless Steel including; 2507 (UNS S32750),

Zeron 100 (UNS S32760), ASTM A890 Gr.5A

2 Passive; not truly

corrosion resistant

904L (UNS N08904)

22% Cr Duplex including; 2205 (UNS S31803/S32205), ASTM

A890 Gr.6A Alloy 825 (UNS N08825)

316L (UNS S31603)

3 Moderate

corrosion

resistance

Copper alloys

Austenitic cast iron

4A Poor corrosion

resistance

Carbon steel

Cast iron

4B Aluminium alloys

4.1.3 Flow assisted corrosion

Erosion-corrosion or other flow assisted corrosion mechanisms of stainless steel are unlikely to occur within the brine circuit

as a direct result of the flow conditions. In seawater at velocities between 1 and 40 m/s stainless steels have been found to be

largely immune to flow assisted corrosion as the flow actually assists with the stabilization of the protective oxide film. The

limited technical data that deals with SWRO brine does not mention either failures or corrosion issues associated with erosion

corrosion for super duplex or super austenitic stainless steel alloys [47]. High nickel and titanium alloys in seawater alsoshow negligible flow assisted corrosion in seawater. Stainless steels, high nickel alloys and titanium are expected to have

similar behaviour in aerated brine solutions to that observed in seawater. However, most copper alloys to some extent are

subject to erosion-corrosion and flow assisted corrosion. Erosion-corrosion can be controlled by limiting the rate of flow to

which the copper alloys are exposed.

4.1.4 Other types of corrosion

In ambient temperature brine stress corrosion cracking is not expected in stainless steel alloys as this mechanism is usually

only observed at temperatures greater than 50C [48]. Microbiologically induced corrosion (MIC) is also unlikely as the pre-


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treatment process removes much of the organic matter in the seawater so the food source for microbial growth is severely

limited.

4.1.5 Corrosion testing in brine solutions

There is limited corrosion test data for alloys in brine solutions. A summary of the accessible information is noted below [41,

45, 49-50].

In brine solutions with a chloride ion concentration of approximately 30,250 ppm:

Titanium and Hastelloy C-276 do not seem to suffer from crevice corrosion

Super-austenitic stainless steel 254 SMO shows signs of minor crevice corrosion in brine though the performance is

similar to that observed in seawater.

In brine solutions containing a chloride ion concentration of 33,410 ppm the following was observed:

Corrosion rate and maximum depth of pitting of 254 SMO, 2205, 904L, Inconel 625, Incoloy 825 in brine are similar

to the rates observed in seawater

Hastelloy C276 and Inconel 625 seem almost immune to crevice corrosion

254SMO and Incoloy 825 display low rates of crevice corrosion.

In brine solutions containing a chloride ion concentration of approximately 40,000ppm:

Nickel base alloy N08367 shows negligible crevice corrosion.


The risk of most forms of corrosion can be minimised through appropriate selection of corrosion resistant alloys. It is

important to select materials that have adequate durability or corrosion resistance for the nominated design life without over

specifying or being too conservative due to cost implications. Where components are readily accessible for maintenance either

due to redundancies in equipment or availability demands allow regular access, then material selection options may includedurability strategies other than the requirement of minimal corrosion over the design life. Such strategies may include the use

of protective coatings or the use of less resistance material with frequent programmed replacements. A life cycle cost analysis

that includes maintenance and replacement costs should be undertaken as part of the alloy selection process.

For flanged components and high integrity seals on valves and pumps it is important to select materials that are resistant to

pitting and crevice corrosion. An indication of pitting resistance is provided by the pitting resistance equivalent number.Experience and research has shown that for service in seawater and concentrated chloride/brine environments it is super-

duplex or super-austenitic stainless steels with PREN>40 that usually have the best resistance. Several of the high nickel

alloys including Hastelloy C-276 and Inconel 625, which also have a PREN>40 also perform well but the cost of these alloys

is usually higher than the stainless steel alloys.

In addition to the requirement for a PREN greater than 40 it is also important that welding is undertaken using codes and

standards prepared specifically for these corrosion resistant alloys. This is to ensure that the corrosion resistance of the weldand the heat affected zone adjacent to the weld has the same corrosion resistance as the parent metal. Poor weld detailing and

finishing will result in zones that have lower durability characteristics than the design requires which will almost certainly be

the sites for early corrosion initiation. It is recommended that all weld oxide scales, welding defects, weld spatter, and surface

irregularities are removed on completion of welding processes. In addition, pickling and passivation should be undertaken

following all welding and fabrication processes to ensure the maximum durability is achieved prior to use of these alloys in the

aggressive brine environment. Pickling and passivation should be performed to ASTM A380 [51].

5. POLYM ERS AND COMPOSITES IN CONTACT WITH BRINE

Economic considerations for the construction of the SWRO brine circuit result in the need to use non-metallic materials where

suitable conditions exist, which include low pressures and low to moderate flow rates. Glass fibre reinforced plastic (GRP)

composites are extensively used for brine piping, as well as components of the outlet system and risers and for components of

the brine circuit clarifiers in many plants. Unreinforced polymers can also be used in contact with brine, for example within

the RO vessels, for pumps and valves components, and many other applications. The selection of unreinforced polymeric and

composite materials is usually based on experience, chemical resistance data provided by materials suppliers or technical

literature.


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In a brine exposure environment, the properties of polymers and GRP composites will change over time predominantly due to

hydrolysis but other factors such as photo-degradation due to UV exposure will also cause degradation. Additionally, contact

with high velocity liquids and entrained particulate matter may cause wear of the resin rich layers and degradation of the GRP.

Polymers are also subject to non-environmentally associated degradation changes as a result of the stresses to which they are

subjected to. These stresses result in the creep and stress relaxation of the polymer materials but these effects taken intoaccount in the design of the component.

Appropriate selection of the resin and glass reinforcement in the GRP, particularly for the corrosion resistant outer layer, canhave a pronounced affect on the resistance to hydrolysis and degradation. As vinyl ester resins have a lower rate of water

absorption than polyester resins, they are usually the resin of choice for the corrosion resistant inner and outer layers as well as

the internal structural layer of GRP components for immersed environments.

RO membranes often fail due to excessive fouling rather than degradation due to contact with brine, so degradation of the

membranes is not discussed further.


Durability of GRP components can be enhanced through the selection of corrosion resistant glass reinforcement and resins

with very low water adsorption in the external corrosion barrier layer of the GRP to limit hydrolysis, for example, vinyl ester

resins. Use of input design strain data with appropriate factors of safety is required to accommodate creep and stress relaxation

over the design life [52]. For components that are also subject to photo-degradation resulting from UV exposure, durability

can be enhanced through the use of either UV stabilisation agents in the outer corrosion barrier layers or the use of protectivecoatings. For applicationswhere particulates may be entrained or at locations where flow rate changes, the application of wear

resistant coatings can enhance the durability of GRP.

The durability requirement of polymers in contact with brine is predominantly based on the selection of polymers with

adequate degradation resistance. Where components are also subject to UV exposure, the UV stabilised versions of thepolymers should be selected. The range of plastics that are reported to have resistance to brine environments include: PTFE,

polyethylene, PVC, polypropylene and PVDF, though the life of these materials will vary and the in-service performance is

usually governed by the quality of the joints, where most failures occur and the quality of the support designs and fastening

systems used. As poorly detailed support and fastening systems can generate high localised stress points that can result in

failure of polymer components.

6.

CONCLUSIONS

This paper aimed to present an overview of the challenges posed by selecting materials for the aggressive SWRO brine created

by desalination plants. Guidelines for material selection based on a combination of literature and experience presented are

summarised below:

For reinforced concrete:

o Understanding the performance characteristics of different cement binders in this environment

o Measures taken to minimise cracking (including undertaking CIRIA C660 analyses)

o Adequate curing (at least 7 days wet curing but preferably 14 days)

o High concentration of SCMs (e.g. 40-70% BFS, 25-35% FA or 7-15% SF by weight of concrete) for

sulphate resistance

o Provision of a sacrificial layer of concrete to account for sulphate attack (in particular magnesium sulphate)o A recognised modelling approach for the prediction of chloride ingress with adequate modelling parameters

(including chloride surface level, chloride diffusion coefficient as measured by an accepted testing method,

chloride threshold for corrosion initiation, maturation coefficient, safety factor)

o Cover requirements as determined by the modelling (or applicable standards if suitable/available)

o Additional protection: coatings, provision for future CP, inhibitors, liners

For metals:

o Alloys with a PREN > 40 for super duplex stainless steel, super austenitic stainless steel or nickel alloys

o Welding undertaken in accordance with the appropriate codes and standards

o Post weld treatment including the removal of weld spatter and surface preparation of welds to remove

surface irregularities

o Post fabrication treatment including pickling and passivation to ASTM A380 [49]

For polymers:o Selection of suitable polymers with established performance

o

For UV exposed components select UV stabilised versions


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o Quality detailing around joints and fastenings is a major factor on service life performance

7. ACKNOWLEDGEMENTS

The authors would like to acknowledge the works of Dr Frank Collins and Dr Marita Berndt. We would like to also thank the

feedback and comments of Miles Dacre and Rob Kilgour, as well as the significant help of Alessandra Mendes.

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9. AUTHOR DETAILS

Frdric Blin is a Principal Engineer in the Advanced Materials Group at AECOM. He

holds a PhD on corrosion inhibitors and has worked on numerous projects, including the

condition assessment of different types of structures exposed to various environments,

non-destructive testing, crack and corrosion monitoring, survey of compliance with

Australian Standards, review and advice on durability issues, technical specification for

infrastructure repair woks, modelling and prediction of future deterioration. He has alsomanaged several projects in the field of civil and transport, especially maritime,

infrastructure, and has authored and co-authored a number of publications, technical

papers and technical reports.

Sarah Furman is a Principal Engineer in the Advanced Materials Group at AECOM.She has a Master of Science in Corrosion Science and Engineering from UMIST inEngland. A materials and corrosion specialist with a broad knowledge of both metallic

and non-metallic materials, she specialises in durability planning for new infrastructure,

performance assessments of materials, materials selection, failure analysis and cathodic

protection design.
http://www.outokumpu.com/http://www.outokumpu.com/

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