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The significance of water chemistry in wet plant operation
Pergamon PII:
Rorlior. P/!ys. Ckm. Vol. S2, Nos I 6, pp. 413-417, 1998 C 1998 Elsevier Science Ltd. All rights reserved Printed m Great Britain SO969-806X(98)00043-7 0969-806X198 Sl9.00 + 0.00
THE SIGNIFICANCE OF WATER CHEMISTRY WET PLANT OPERATION
TN
DAVID COPPELL Fuston
Associates
UK Limited: 23 Woodland Aye. Wmdsor. Be&s. SL4 4AG: UK (Consultant to PURIDEC h-radiation Technologies)
ABSTRACT The use of cobalt radiation sources in a wet storage environment requires care in the control of that environment to avoId the potential for deterioration of the steel. This paper describes the factors which are important and recommends a set of water quality conditions. It also describes the measures which PURIDEC has taken to monitor the performance of product in service and outlines some of the results from that surveillance programme.
KEYWORDS Co-60. radiation sources. corrosion, sweillance.
pitting. silica.
INTRODUCTION The use of radiation sources in wet storage conditions is relatively world and requires attention to some environmental factors which Cobalt-60 radiation sources are encapsulated in stainless steel and this water environments. The focus on water chemistry is intended to avoided.
unusual are not material ensure
PURIDEC provides help and support to enable users to avoid situations equipment are exposed to adverse conditions. SIGNIFICANT
in the radiation sources relevant in dry storage. can deteriorate in certain that these situations are
in which sources in their
FACTORS
The interaction between stainless steels and aqueous environments remains an uncertain science and research continues in this field in an attempt to bring tirther clarity. Within a cobalt storage pool, the situation is made more complex by thermal cycling and by the radiolysis products from irradiation both of water and of materials dissolved or suspended in it. The demineralised water normally used in storage pools is usually an effective and secure medium in which to store stainless steel. However, there are certain constituents and contaminants in the water which can affect this.Ozone. This material is generated by the radiation fields which surround Co-60 radiation sources in service Ozone is a strong oxidising agent and, when combined with other materials, is capable of causing oxidation of stainless steel (NACE; I997), Peroxides. These are formed by irradiation of water. They are powerful oxidising agents and can lead to oxidation of stainless steel when combined with other materials. Halides (or halogens). Halides (for example, chloride ions) are sometimes present as contaminants in pool water. Halides too are oxidising agents and can also act as de-passivating agents. They are the most common initiators for pitting corrosion. Nitrates. Can be formed from the radiolysis of dissolved nitrogen and oxygen. Nitrates are oxidising agents and, though they do not initiate pitting corrosion, will accelerate its progress once initiated by some other factor.
413
414
David Coppell
Ferric ions Ferric ions occasionally arise as corrosion products from steel They act as additional oxidising agents and can accelerate pit growth once pitting has been initiated. Silica and insolubles. Silica and other insoluble materials can be deposited from water as surface layers on the radiation sources This may have two effects; first it masks the real surface appearance of the steel; second and more significantly, it can create a micro-environment trapped within the surface layer or between it and the steel The composition of the micro-environment might differ significantly from the bulk composition of the pool water. Organic Material and Insoluble Carbon. Insoluble carbon has been identified as a potential initiator for crevice corrosion. Organic material degrades in the presence of high radiation fields and also in the presence of ozone and can create deposits of insoluble carbon. pH. This has a small but definite effect on the tendency for pitting corrosion. In most circumstances, alkaline environments (especially those at pH >9) increase the effectiveness of the passive layer on the surface of stainless steels whilst acid environments can damage or break down the layer in some cases. However, there is also evidence that both strongly alkaline and strongly acid environments can encourage pitting corrosion. In general, an approximately neutral pH is recommended. Conductivity. This is used as a measure of the extent to which ionic species are present in the pool water. Though there is no exact correlation, it may be assumed to be an indication of the potential presence of chloride or other halide species which would be detrimental.
CORROSION MECHANISMS FOR STAINLESS STEEL IN WATER ENVIRONMENTS
Stainless steels of the 316L or 317L families have been designed to be able to cope in environments such as storage pools and are not prone to general corrosion in such situations. Under normal circumstances, the steel operates in the passive region of its electrochemical polarization curve. It is covered by a thin but dense layer of passive mixed metal oxides which provide it with protection. There are, however, certain stimuli which can cause this passivity to fail.
Pitting ('orrosion. Pitting corrosion occurs where a steel surface is preferentially attacked in one or more local positions. It is a particularly unwelcome type of corrosion because it can lead to perforation of the steel in a relatively short period of time Pitting corrosion is usually initiated by either halides (eg chloride at a level of 50ppm or above) or by imperfections in the steel surface (eg sulphide inclusions or carbides formed during a welding process) Sedriks 1996). Following initiation, pitting can be self-sustaining For example, if pitting is initiated by a local presence of excess chloride ions, the process may continue even after the chloride has been reduced to trace quantities. The reason for this is that the high current density in the base of the pit results in a strong positive charge which, in turn, attracts negatively charged chloride ions and other anions and so concentrates them in the area of the pit. Pitting is one of the most common causes of failure of stainless steel components in environments similar to those found in cobalt storage pools.
Crevice Corrosion Crevice corrosion may occur at positions where the environment is not typical of the general environment in the pool. For example, where radiation sources are clamped into modules or frames, there may be crevices formed at the point where the metals are in close contact. Crevices present opportunities for the concentration of otherwise dilute species (eg chloride), especially during the cyclical heating/drying process which occurs when the source rack is repeatedly removed from and replaced in the pool.
10th International Meeting on Radiation Processing
415
The concentration of trace materials from the water in crevices may lead to the initiation of corrosion sites even in situations where the bulk pool water environment would not support the initiation of corrosion
Intergranular Corrosion Stainless steels are crystalline in structure; the crystals are referred to as "grains". The boundary regions between grains normally have a very similar composition to bulk material and so have the same level of corrosion resistance. However, if the steel is subjected to inappropriate thermal treatment during its service life, the composition of the grain boundary regions can change. The most significant effect is that chromium is absorbed in the formation of chromium carbides and the consequent depletion of free chromium near to the grain boundary results in it losing some of the resilient properties of stainless steel. This process is called sensitisation. In this situation, grain boundaries become more vulnerable to a form of corrosion called intergranular corrosion. Intergranular corrosion is usually a slow process which does not lead to rapid penetration of the steel as can happen with pitting corrosion but, ultimately, intergranular corrosion might weaken parts of the structure.
THE EFFECTS OF CHLORIDE (AND OTHER HALIDES AND HALOGENS) The presence and concentration of chloride and other chlorine-containing species is the most common and often the most important factor in the initiation of pitting corrosion. Chloride concentrations as low as 50 ppm at the metal surface, can be significant in the initiation of pitting. At levels of 50ppm and above, chloride ions compete for positions within the passive layer on the surface of the steel. They create sites at which the layer ceases to have the structure of a hydrated metal oxide "gel" but, instead, contains regions of metal chloride which then dissolves leaving localised access through the passive layer direct to the metal surface. The overall effect of chloride is to act as a local depassivating agent as well as an oxidising agent.
MICRO-ENVIRONMENTS The features of the bulk pool water and its effect on the behaviour of steel may be significantly changed if there are other factors at work which create localised, atypical environments. An example of this would be crevice corrosion although both cobalt radiation sources and the hardware in which they are stored and used are designed so as to minimise the potential for crevice corrosion. A similar effect can be created by the deposition of insoluble layers on the surface of steel during service An example of this is the deposition of silica layers from colloidal Si02 in the water. Silica has been found to form hard, strongly-adhering coatings which are built up in strata. The deposition of carbon on the surface of the steel may have a similar effect; the carbon can be generated by the irradiation and degradation of organic material (for example oil or grease) in the pool water. Periodically, parts of the coating may break away (probably as a result of differential thermal expansion during thermal cycling) and the result can be the creation of a rough or layered surface topography. This may have a similar effect to a crevice by providing the opportunity for localised concentration of species such as chloride. In addition, the deposition process which results in the formation of such insoluble layers may combine with the thermal cycling and drying process to trap and concentrate trace materials within the matrix; for example chloride ions.
416
David Coppell
Either of these mechanisms can lead to an increase in concentration of some species by at least a factor of 5 by comparison with the bulk pool water. This may be one of the most effective and yet insidious mechanisms for the initiation of pitting corrosion on the surface of cobalt sources in wet irradiators There is clear evidence for it having occurred in at least one cobalt storage pool.
THE PURIDEC SPECIFICATION FOR WATER QUALITY The specification currently recommended by PURIDEC is for the following water conditions:total halide concentration not to exceed 10ppm, this is intended to avoid the potential for concentration effects such as crevices or silica layers leading to the creation of concentrations exceeding 50ppm and the consequent risk of pitting corrosion, pH to be in the range 5.5 to 8.5. keeping the pH in this range minimises the risk of damage to the passive oxide layer on the surface of the stainless steel source capsules, conductivity to be less than l O00~tS/m conductivity may be used as a measure of the overall concentration of ionic species in the water environment; though not as specific as monitoring for individual species (eg chloride), it has been shown to be well correlated with risk factors in water quality, In addition, water should be analysed regularly (at least every six months) to determine what impurities and trace materials it contains. If unusual impurities are identified, expert advice should be sought to determine whether these might constitute a hazard to the integrity of the radiation sources and other components of the irradiator system, THE PURIDEC SURVEILLANCE P R O G R A M M E The objectives of PURIDEC's surveillance programme are to:* help plant operators to ensure that the water quality in their facilities is consistent and remains within the optimum range for protection of the stainless steel radiation sources and other components within the plant, • monitor the condition and performance of radiation sources in individual plants and to provide reassurance to the plant operators, • gather information to share with the radiation sources and gamma sterilisation industries and to use for continuous improvement in the design of radiation sources, Source users are asked to agree to participation in the programme which includes the following elements:* routine pool water sampling and analysis (minimum once per year), • review of plant operating records and operating environment, • non-destructive examination of sources in situ (minimum every two years), • destructive testing/analysis of sample sources (minimum every five years),
RESULTS FROM WATER QUALITY SURVEILLANCE During the past three years, PURIDEC has collected more than 70 samples of water from a variety of cobalt storage pools from around the world which contained sources supplied by PURIDEC and other manufacturers. Some examples of these results are:•
12% of the samples showed chloride levels in excess of the PURIDEC recommended specification of 10ppm PURIDEC has examined the situation carefully in these plants. It has satisfied itself that
10th International Meeting on Radiation Processing
417
there is no significant risk to the sources and continues to work with the plant operators to reduce the level of chloride in the water. 53% of the samples showed a value for pH which was beneath the PURIDEC recommended lower limit o f 5.5. The great majority of samples which fell outside the recommended range were not grossly acid. In these cases, the effects o f pH on steel passivity are not great, especially where the water is fairly pure as in these cases. There were no other significant indications of risk and it was concluded that, whilst there may be benefits in raising the pH, the risk represented by the current levels is tolerable. More than 25% o f the samples showed conductivity levels in excess o f P U R I D E C ' s recommended upper limit o f 1000 btS/m. After closer examination, corrective action was taken in those cases where the high conductivity had arisen from the presence of undesirable impurities. More than 30% of the samples contained silica levels in excess o f 10ppm At present, there is insufficient data available for PURIDEC to recommend a specification for maximum silica levels but it is clear that, in several examples, there was sufficient silica available in the pool water to lead to the formation o f deposits on the sources with the attendant risk of concentration effects. FUTURE INITIATIVES Water Qualtty - Continuous Improvement
At PURIDEC, there will be continued focus on water quality and a managed programme to control some of the unwanted contaminants so as to minimise the risk. Surveillance
The benefits of an organised surveillance problem have been very clear. The future will bring more data and more understanding from this programme and will assist with optimising the source design and water specification as well as verifying that radiation sources in service are performing to the required level. New Materials
PURIDEC continually evaluates and tests alternative source construction materials. Though there is no immediate prospect for change away from 316L or 317L stainless, there seems clear evidence that materials exist which could provide even more resistance to water impurities as well as having the potential to provide other benefits such as improved heat resistance. The challenges with these alternatives are manufacturability and compatibility with component materials currently in service.
REFERENCES NACE 1997
The Effect o f Dissolved Ozone on the Corrosion Behaviour o f Stainless Steel in Artificial Sea Water - paper 436 - Corrosion 97 - NACE; Houston,
Sedriks 1996
Corrosion of Stainless Steels - A John Sedriks; Wiley Interscience; 1996
Rorlior. P/!ys. Ckm. Vol. S2, Nos I 6, pp. 413-417, 1998 C 1998 Elsevier Science Ltd. All rights reserved Printed m Great Britain SO969-806X(98)00043-7 0969-806X198 Sl9.00 + 0.00
THE SIGNIFICANCE OF WATER CHEMISTRY WET PLANT OPERATION
TN
DAVID COPPELL Fuston
Associates
UK Limited: 23 Woodland Aye. Wmdsor. Be&s. SL4 4AG: UK (Consultant to PURIDEC h-radiation Technologies)
ABSTRACT The use of cobalt radiation sources in a wet storage environment requires care in the control of that environment to avoId the potential for deterioration of the steel. This paper describes the factors which are important and recommends a set of water quality conditions. It also describes the measures which PURIDEC has taken to monitor the performance of product in service and outlines some of the results from that surveillance programme.
KEYWORDS Co-60. radiation sources. corrosion, sweillance.
pitting. silica.
INTRODUCTION The use of radiation sources in wet storage conditions is relatively world and requires attention to some environmental factors which Cobalt-60 radiation sources are encapsulated in stainless steel and this water environments. The focus on water chemistry is intended to avoided.
unusual are not material ensure
PURIDEC provides help and support to enable users to avoid situations equipment are exposed to adverse conditions. SIGNIFICANT
in the radiation sources relevant in dry storage. can deteriorate in certain that these situations are
in which sources in their
FACTORS
The interaction between stainless steels and aqueous environments remains an uncertain science and research continues in this field in an attempt to bring tirther clarity. Within a cobalt storage pool, the situation is made more complex by thermal cycling and by the radiolysis products from irradiation both of water and of materials dissolved or suspended in it. The demineralised water normally used in storage pools is usually an effective and secure medium in which to store stainless steel. However, there are certain constituents and contaminants in the water which can affect this.Ozone. This material is generated by the radiation fields which surround Co-60 radiation sources in service Ozone is a strong oxidising agent and, when combined with other materials, is capable of causing oxidation of stainless steel (NACE; I997), Peroxides. These are formed by irradiation of water. They are powerful oxidising agents and can lead to oxidation of stainless steel when combined with other materials. Halides (or halogens). Halides (for example, chloride ions) are sometimes present as contaminants in pool water. Halides too are oxidising agents and can also act as de-passivating agents. They are the most common initiators for pitting corrosion. Nitrates. Can be formed from the radiolysis of dissolved nitrogen and oxygen. Nitrates are oxidising agents and, though they do not initiate pitting corrosion, will accelerate its progress once initiated by some other factor.
413
414
David Coppell
Ferric ions Ferric ions occasionally arise as corrosion products from steel They act as additional oxidising agents and can accelerate pit growth once pitting has been initiated. Silica and insolubles. Silica and other insoluble materials can be deposited from water as surface layers on the radiation sources This may have two effects; first it masks the real surface appearance of the steel; second and more significantly, it can create a micro-environment trapped within the surface layer or between it and the steel The composition of the micro-environment might differ significantly from the bulk composition of the pool water. Organic Material and Insoluble Carbon. Insoluble carbon has been identified as a potential initiator for crevice corrosion. Organic material degrades in the presence of high radiation fields and also in the presence of ozone and can create deposits of insoluble carbon. pH. This has a small but definite effect on the tendency for pitting corrosion. In most circumstances, alkaline environments (especially those at pH >9) increase the effectiveness of the passive layer on the surface of stainless steels whilst acid environments can damage or break down the layer in some cases. However, there is also evidence that both strongly alkaline and strongly acid environments can encourage pitting corrosion. In general, an approximately neutral pH is recommended. Conductivity. This is used as a measure of the extent to which ionic species are present in the pool water. Though there is no exact correlation, it may be assumed to be an indication of the potential presence of chloride or other halide species which would be detrimental.
CORROSION MECHANISMS FOR STAINLESS STEEL IN WATER ENVIRONMENTS
Stainless steels of the 316L or 317L families have been designed to be able to cope in environments such as storage pools and are not prone to general corrosion in such situations. Under normal circumstances, the steel operates in the passive region of its electrochemical polarization curve. It is covered by a thin but dense layer of passive mixed metal oxides which provide it with protection. There are, however, certain stimuli which can cause this passivity to fail.
Pitting ('orrosion. Pitting corrosion occurs where a steel surface is preferentially attacked in one or more local positions. It is a particularly unwelcome type of corrosion because it can lead to perforation of the steel in a relatively short period of time Pitting corrosion is usually initiated by either halides (eg chloride at a level of 50ppm or above) or by imperfections in the steel surface (eg sulphide inclusions or carbides formed during a welding process) Sedriks 1996). Following initiation, pitting can be self-sustaining For example, if pitting is initiated by a local presence of excess chloride ions, the process may continue even after the chloride has been reduced to trace quantities. The reason for this is that the high current density in the base of the pit results in a strong positive charge which, in turn, attracts negatively charged chloride ions and other anions and so concentrates them in the area of the pit. Pitting is one of the most common causes of failure of stainless steel components in environments similar to those found in cobalt storage pools.
Crevice Corrosion Crevice corrosion may occur at positions where the environment is not typical of the general environment in the pool. For example, where radiation sources are clamped into modules or frames, there may be crevices formed at the point where the metals are in close contact. Crevices present opportunities for the concentration of otherwise dilute species (eg chloride), especially during the cyclical heating/drying process which occurs when the source rack is repeatedly removed from and replaced in the pool.
10th International Meeting on Radiation Processing
415
The concentration of trace materials from the water in crevices may lead to the initiation of corrosion sites even in situations where the bulk pool water environment would not support the initiation of corrosion
Intergranular Corrosion Stainless steels are crystalline in structure; the crystals are referred to as "grains". The boundary regions between grains normally have a very similar composition to bulk material and so have the same level of corrosion resistance. However, if the steel is subjected to inappropriate thermal treatment during its service life, the composition of the grain boundary regions can change. The most significant effect is that chromium is absorbed in the formation of chromium carbides and the consequent depletion of free chromium near to the grain boundary results in it losing some of the resilient properties of stainless steel. This process is called sensitisation. In this situation, grain boundaries become more vulnerable to a form of corrosion called intergranular corrosion. Intergranular corrosion is usually a slow process which does not lead to rapid penetration of the steel as can happen with pitting corrosion but, ultimately, intergranular corrosion might weaken parts of the structure.
THE EFFECTS OF CHLORIDE (AND OTHER HALIDES AND HALOGENS) The presence and concentration of chloride and other chlorine-containing species is the most common and often the most important factor in the initiation of pitting corrosion. Chloride concentrations as low as 50 ppm at the metal surface, can be significant in the initiation of pitting. At levels of 50ppm and above, chloride ions compete for positions within the passive layer on the surface of the steel. They create sites at which the layer ceases to have the structure of a hydrated metal oxide "gel" but, instead, contains regions of metal chloride which then dissolves leaving localised access through the passive layer direct to the metal surface. The overall effect of chloride is to act as a local depassivating agent as well as an oxidising agent.
MICRO-ENVIRONMENTS The features of the bulk pool water and its effect on the behaviour of steel may be significantly changed if there are other factors at work which create localised, atypical environments. An example of this would be crevice corrosion although both cobalt radiation sources and the hardware in which they are stored and used are designed so as to minimise the potential for crevice corrosion. A similar effect can be created by the deposition of insoluble layers on the surface of steel during service An example of this is the deposition of silica layers from colloidal Si02 in the water. Silica has been found to form hard, strongly-adhering coatings which are built up in strata. The deposition of carbon on the surface of the steel may have a similar effect; the carbon can be generated by the irradiation and degradation of organic material (for example oil or grease) in the pool water. Periodically, parts of the coating may break away (probably as a result of differential thermal expansion during thermal cycling) and the result can be the creation of a rough or layered surface topography. This may have a similar effect to a crevice by providing the opportunity for localised concentration of species such as chloride. In addition, the deposition process which results in the formation of such insoluble layers may combine with the thermal cycling and drying process to trap and concentrate trace materials within the matrix; for example chloride ions.
416
David Coppell
Either of these mechanisms can lead to an increase in concentration of some species by at least a factor of 5 by comparison with the bulk pool water. This may be one of the most effective and yet insidious mechanisms for the initiation of pitting corrosion on the surface of cobalt sources in wet irradiators There is clear evidence for it having occurred in at least one cobalt storage pool.
THE PURIDEC SPECIFICATION FOR WATER QUALITY The specification currently recommended by PURIDEC is for the following water conditions:total halide concentration not to exceed 10ppm, this is intended to avoid the potential for concentration effects such as crevices or silica layers leading to the creation of concentrations exceeding 50ppm and the consequent risk of pitting corrosion, pH to be in the range 5.5 to 8.5. keeping the pH in this range minimises the risk of damage to the passive oxide layer on the surface of the stainless steel source capsules, conductivity to be less than l O00~tS/m conductivity may be used as a measure of the overall concentration of ionic species in the water environment; though not as specific as monitoring for individual species (eg chloride), it has been shown to be well correlated with risk factors in water quality, In addition, water should be analysed regularly (at least every six months) to determine what impurities and trace materials it contains. If unusual impurities are identified, expert advice should be sought to determine whether these might constitute a hazard to the integrity of the radiation sources and other components of the irradiator system, THE PURIDEC SURVEILLANCE P R O G R A M M E The objectives of PURIDEC's surveillance programme are to:* help plant operators to ensure that the water quality in their facilities is consistent and remains within the optimum range for protection of the stainless steel radiation sources and other components within the plant, • monitor the condition and performance of radiation sources in individual plants and to provide reassurance to the plant operators, • gather information to share with the radiation sources and gamma sterilisation industries and to use for continuous improvement in the design of radiation sources, Source users are asked to agree to participation in the programme which includes the following elements:* routine pool water sampling and analysis (minimum once per year), • review of plant operating records and operating environment, • non-destructive examination of sources in situ (minimum every two years), • destructive testing/analysis of sample sources (minimum every five years),
RESULTS FROM WATER QUALITY SURVEILLANCE During the past three years, PURIDEC has collected more than 70 samples of water from a variety of cobalt storage pools from around the world which contained sources supplied by PURIDEC and other manufacturers. Some examples of these results are:•
12% of the samples showed chloride levels in excess of the PURIDEC recommended specification of 10ppm PURIDEC has examined the situation carefully in these plants. It has satisfied itself that
10th International Meeting on Radiation Processing
417
there is no significant risk to the sources and continues to work with the plant operators to reduce the level of chloride in the water. 53% of the samples showed a value for pH which was beneath the PURIDEC recommended lower limit o f 5.5. The great majority of samples which fell outside the recommended range were not grossly acid. In these cases, the effects o f pH on steel passivity are not great, especially where the water is fairly pure as in these cases. There were no other significant indications of risk and it was concluded that, whilst there may be benefits in raising the pH, the risk represented by the current levels is tolerable. More than 25% o f the samples showed conductivity levels in excess o f P U R I D E C ' s recommended upper limit o f 1000 btS/m. After closer examination, corrective action was taken in those cases where the high conductivity had arisen from the presence of undesirable impurities. More than 30% of the samples contained silica levels in excess o f 10ppm At present, there is insufficient data available for PURIDEC to recommend a specification for maximum silica levels but it is clear that, in several examples, there was sufficient silica available in the pool water to lead to the formation o f deposits on the sources with the attendant risk of concentration effects. FUTURE INITIATIVES Water Qualtty - Continuous Improvement
At PURIDEC, there will be continued focus on water quality and a managed programme to control some of the unwanted contaminants so as to minimise the risk. Surveillance
The benefits of an organised surveillance problem have been very clear. The future will bring more data and more understanding from this programme and will assist with optimising the source design and water specification as well as verifying that radiation sources in service are performing to the required level. New Materials
PURIDEC continually evaluates and tests alternative source construction materials. Though there is no immediate prospect for change away from 316L or 317L stainless, there seems clear evidence that materials exist which could provide even more resistance to water impurities as well as having the potential to provide other benefits such as improved heat resistance. The challenges with these alternatives are manufacturability and compatibility with component materials currently in service.
REFERENCES NACE 1997
The Effect o f Dissolved Ozone on the Corrosion Behaviour o f Stainless Steel in Artificial Sea Water - paper 436 - Corrosion 97 - NACE; Houston,
Sedriks 1996
Corrosion of Stainless Steels - A John Sedriks; Wiley Interscience; 1996