The control of corrosive conditions caused by concentration of low-volatility solutes in boilers and steam generators

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1 P R E P R I N T ICPWS XV Berlin, September 8 11, 2008 The control of corrosive conditions caused by concentration of low-volatility solutes in boilers and steam generators Geoffrey J Bignold GJB Chemistry for Power Ltd, UK. geoff.bignold@gjbchemforpower.co.uk The risk of forming concentrated solutions of solutes in boilers is a consequence of the boiling process. Corrosion can ensue whenever an acidic concentrate is formed. To control this risk, drum boilers are dosed to achieve mildly alkaline conditions. However, whenever very high concentration factors occur, it is also possible to generate highly corrosive alkaline conditions. The key parameter is the concentration of hydroxide ion that can be reached. All solutes contribute to the elevation of boiling point of water. The limit to concentration processes in boilers is controlled by the sum of all ion and non-volatile solutes that are present. Thus the OH - concentration is limited by the presence of other anions. Alkali dosing is most beneficial in boilers that frequently have contaminants such as chlorides in the boiler water. In such plants, it suppresses the risk of acidic forms of corrosion without there being excessive risk of caustic attack. By contrast, in plants that are free of other contaminants the risk of caustic attack may be greater. Phosphate dosing regimes can generate high levels of hydroxide ion concentration in zones of solute accumulation. However, the phosphates themselves limit the ultimate OH - concentration that can be reached. The case is made for controlling the dosing of a low-volatility alkali to a drum boiler so that it is just sufficient to control the risk of acid corrosion, without giving a risk of caustic attack. Introduction On-load corrosion failures in drum boilers have long been recognised as a result of the concentration of non-volatile solutes stimulated by the boiling process [1]. The concentration process effectively occurs in two stages: In the first stage any contaminants that are present in the feedwater accumulate in the bulk boiler water as steam is generated. A mass balance becomes established between the rates of supply of solute from the feedwater to the boiler and the loss of these solutes via boiler blow-down together with any carry-over of solutes into the steam. The second stage involves the formation of the much more concentrated and potentially corrosive solutions within porous deposits on the bore surfaces of the boiler tubes and within any surface defects. Studies of the influence of artificial defects in tube bore surfaces on concentration processes occurring during boiling have confirmed that there is a limit to the concentration reached. The limiting factor is the reduction of vapour pressure of the concentrated liquid, which ultimately leads to the suppression of boiling [2]. Thus boiling continues to stimulate the concentration process until the boiling point of the resulting solution matches the local elevation of temperature above the boiling point of pure water.. In practice, provided good control over the thickness of porous deposits within boilers is maintained, the concentration factor can be constrained (this is the justification for periodic chemical cleaning of boilers). Nonetheless, it is possible for concentration factors between nonboiling liquid in porous deposits and bulk boiler water to reach at least 10,000 times. The corrosion risk, being much greater for acidic concentrates than for alkaline materials, is controlled by the addition of alkaline dosing reagents. Volatile alkalis (ammonia and amines) offer little protection to the boiler because they are weakly dissociated. Ammonia and the less hydrophilic amines favour the steam phase and will not accumulate in porous deposits. Thus the use of all volatile treatment, AVT, must be confined to

2 plants in which there is very good control of contaminants. It becomes necessary to use a non-volatile alkali in those drum boiler plants in which the risk of contaminant ingress is high. In general this applies to older plants were there may be increasing occurrence of condenser leakage and where there is no condensate polishing plant available. Power plants at coastal and estuarine locations are particularly likely to require the use of non-volatile alkalis in the boiler. Whereas phosphate dosing has historically been favoured in many countries, the use of sodium hydroxide has also been widely applied and is covered in authoritative guidelines ([3] and [4]). It is acknowledged that the use of too much sodium hydroxide itself leads to a corrosion risk (often referred to as caustic gouging) and in highly stressed components there may be a possibility of stress corrosion cracking. The alternative phosphate dosing is also associated with some problems such as acidic phosphate corrosion [5], corrosion fatigue [6] and phosphate hide-out, which can hinder chemistry control in high pressure boilers. Criteria for avoidance of alkaline corrosion Ultimately, alkaline corrosion processes can be controlled by ensuring that the hydroxide ion concentration is not allowed to rise excessively. It is convenient, in this context, to express this in terms of where: = - log 10 (a OH- ) a OH- is the hydroxide ion activity and in low concentrations approximates to the hydroxide ion concentration. Experimental studies of caustic corrosion processes have, in general, only shown unacceptable rates at concentrations above 1 molal (i.e. 0) [7]. The aim in using non-volatile alkalis for boiler alkalisation should therefore be to ensure that concentration processes cannot lead to the development of zones where the hydroxide ion activity exceeds this value, i.e. where is negative. Given good data for the ionic equilibrium constants of the relevant reactions, it is possible using iterative computation techniques to perform calculations of speciation equilibria from which can be evaluated. The calculations reported in this paper address the equilibria of the hydroxide, phosphate and chloride anions in the presence of sodium cations. Similar iterative procedures are used in some commercially available codes, enabling validation of the calculation method. Concentration of mixtures by boiling In this paper consideration will be given to the concentration by boiling of mixtures of common boiler contaminants and non-volatile alkaline dosing agents. As a worst case it is necessary to consider a boiler with significant porous deposits in the bore of evaporator tubes and with sufficient heat flux to drive the solute concentration process. The equilibrium condition is reached when the overall concentration of all solutes at the base of the pores is sufficient to raise the local boiling point to match the temperature elevation at the metal/deposit interface. NaOH without other impurities For the simple case of sodium hydroxide dosed boiler water with no contaminants, it is merely necessary to consider the concentration of NaOH itself. A bulk concentration of 2 mg.kg -1 as NaOH is equivalent to Molal and has a of 4.3. As, at temperatures above 318ºC (melting point) this is fully miscible with water, the relationship between the concentration achieved by boiling and the of the concentrated liquid phase is simple: Concentration (Molal) Figure 1: Drop in caused by concentrating boiler water dosed only with NaOH (no contaminating species present) as boiling occurs at the base of porous deposits. Figure 1 demonstrates that: 2

3 a) When NaOH is the only solute in the boiler water, it is feasible to generate concentrated liquids with <0. b) This can only occur if the combination of porous deposits and heat flux is sufficient to generate a concentration factor in excess of 50,000 times. The latter point is the key to the successful use of sodium hydroxide dosing in boilers where excessive porous deposits are not allowed to develop and heat flux is relatively low. NaOH in the presence of strongly ionised impurities When alkaline boiler dosing is used to protect against the threat from sea water ingress to the circuit the main contaminant to consider is NaCl. Clearly, minor constituents such as magnesium chloride and sulphate play a role in determining the ph and but their influence is secondary to the dominant species. (In practice the acid hydrolysing species such as magnesium chloride will, of course cause a reduction in the drop of.) - NaOH alone NaOH with NaCl contamination Total Concentration (Molal) Figure 2: Drop in caused by concentrating boiler water dosed with NaOH at 2 mg.kg -1 as NaOH in the presence of 1.33 mg.kg -1 of NaCl. As the temperature is raised these equilibria shift to the right so that in boiler waters dosed with Na 3 PO 4 the dominant anion is H 2 PO 4 - and for every 3 dosed sodium ions there are two hydroxyl ions [8] see also figure 3. This means that, on a mole for mole basis, tri-sodium phosphate yields twice as much hydroxyl ion as sodium hydroxide. The basis behind the use of phosphates in boilers is, in part, their action as buffers, stabilising the ph in the presence of contaminants. As phosphates become concentrated in porous deposits the hydrolysis equilibrium shifts in favour of the 2- HPO 4 ion. Thus, in concentrated solutions there remains approximately one hydroxyl ion for every three dosed sodium ions and the solution remains capable of sustaining a very low. Ion Concentration (M/kg) 1.E-03 1.E-04 1.E-05 1.E-06 Neutral H 2 PO 4 - H 3 PO 4 HPO E ph at 300C Figure 3: Phosphate speciation diagram for boiler water at 300ºC based on data from ref [8]. Figure 4 shows the calculated for boiler waters dosed with Na 3 PO 4 and with Na 2.6 H 0.4 PO 4 compared with NaOH on the basis of equal levels of sodium ion. Figure 2 shows that the risk of forming solutions with <0 is quite significantly lower in a contaminated boiler with NaOH dosing than in a boiler that is free of contamination. NaOH Na 2.6H 0.4PO 4 Na 3PO 4 Na 3 PO 4 without other impurities Phosphate systems provide alkalinity by hydrolysis of the phosphate anion:. PO H 2 O HPO OH - + H 2 O H 2 PO OH Sodium Concentration (Molal) Figure 4: Drop in caused by concentrating boiler water dosed with Na 3 PO 4 and with Na 2.6 H 0.4 PO 4 compared with NaOH (for equal sodium ion concentrations). Calculated using the data for phosphate equilibria at 300ºC from ref [8]. 3

4 Thus it is seen that the risk of forming solutions with <0 from phosphate dosed boiler waters is, as expected, less than for NaOH dosed water, but not significantly different from NaOH when other contaminants are present. Implications for practical boiler dosing For any boiler plant the targets for dosing the water should be optimised to control: a) the risks of corrosion arising as a result of impurity ingress to the circuit, b) the solubilisation and transport around the circuit of oxidised products of the materials of construction, c) the threat of consequential damage resulting from overdosing. The achievement of mildly alkaline conditions minimises the solubility of the oxides of iron and other transition metals used in the alloys from which steam/water circuits are built. Sufficient alkali is necessary to suppress any formation of any localised acidic regions as a result of ingress of impurities to the circuit. The optimum dosing uses enough alkali to achieve this, with a small margin of safety. The use of excess would increase the risk that very low conditions may develop and lead to caustic corrosion. Attaining this optimum dosing level, where the added alkali is just sufficient for purpose, presents a greater challenge for phosphate dosed boilers than for NaOH dosed plants. In the latter case an immediate, albeit non-specific, indication of the presence of contaminants can be obtained by measuring the conductivity after cation exchange. This simple on-line measurement can readily be used to alert operators to the need to adjust the dosing rate and to blow down the boiler in response to the accumulation of contaminants in the boiler water. By contrast, when phosphate dosing is applied the phosphate ion itself negates the value of measuring conductivity after cation exchange. It becomes necessary to undertake further analysis to quantify the concentrations of chloride, sulphate and other contaminants. If these measurements are performed off-line on batch samples it implies that there will be an additional delay in response to the presence of contaminants. Thus, a higher concentration of phosphate may be necessary to provide a safety margin to accommodate this delay. The wealth of worldwide experience shows that those drum boiler plants that are subject to the risk of impurity ingress to the steam water circuits and are therefore not suitable for AVT chemistry can be successfully operated with either phosphate or hydroxide dosing. Both regimes have their advocates and the findings of this study alone will not alter their views on their relative merits. In high pressure plant it is simpler to achieve stable control of conditions using hydroxide dosing [9], whereas phosphate chemistry can be more tolerant of overdosing in lower pressure cycles. Conclusions Concentrated solutions of solutes occur in boilers as a direct consequence of the boiling process. To control the resulting corrosion risk, drum boilers are dosed to achieve mildly alkaline conditions. However, whenever very high concentration factors occur, it is also possible to generate highly corrosive alkaline conditions. The key parameter is the concentration of hydroxide ion that can be reached. The risks of alkaline corrosion, which may become significant if simple hydroxides (e.g. NaOH) are over-dosed, are lessened whenever there are other contaminants present. It follows that optimum dosing of NaOH should ideally be linked to the actual concentration of contaminants. Phosphate dosing, although yielding less risk of alkaline corrosion, is less amenable to control as contamination levels vary, and therefore should be applied with a larger margin to protect against contaminant ingress events. Literature [1] G M W Mann: History and causes of on-load waterside corrosion in power boilers. British Corrosion Journal 12(1) 6-14 (1977). [2] C B Ashmore, M H Hurdus, A P Mead, P J B Silver, L Tomlinson and D J Finnigan., Concentrating effects of simulated defects 4

5 under boiling conditions in high temperature water. Corrosion, NACE, 44(6) (1988). [3] VGB PowerTech Guidelines for feed water, boiler water and steam quality for power plants / industrial plants, VGB-R 450 Le (2004). [4] EPRI Cycle Chemistry guidelines for fossil plants: phosphate continuum and caustic treatment. EPRI (2004). [5] B Dooley and W P McNaughton, Appropriate controls for phosphate boiler water treatments to avoid acid phosphate corrosion and hydrogen damage. PowerPlant Chemistry, 3(3), (2001). [6] J Stodola, Fifteen years of equilibrium phosphate treatment (correct use of phosphate in drum boilers. PowerPlant Chemistry, 5(2), (2003). [7] W M M Huijbregts, Corrosion of unalloyed steels in different alkaline solutions at high temperatures and under high pressures. KEMA Scientific & Technical Reports 1(1): 1-9. [8] R E Mesmer and C F Baes, Phosphoric acid dissociation equilibria in aqueous solutions to 300 C. Journal of Solution Chemistry, 3(4), (1974). [9] G J Verib, Conversion of a drum boiler from phosphate to caustic treatment. Proc. Eighth Int. Conf. on Cycle Chemistry in fossil and combined cycle plants with HRSGs. Calgary. EPRI (2006). 5