Biocorrosion Risk Assessment

Transcription

1 Biocorrosion Risk Assessment Andrew M.Pritchard European Summer School on Biologically Influenced Corrosion, University of Portsmouth 7-13 July 2002

2 Contents T Management and assessment of risks T A definition of MIC T Biofilms: their importance and our ignorance T Risk: the essential conditions for MIC T Risk: where MIC is found again and again T Risk: construction of a database of MIC T Aspects of MIC T Experience so far with the database T Conclusions

3 Risk management 1 - overall risk T Major concern and duty for managers T Objective: reduction of danger or overall risk to a minimum (ALARP = As Low As Reasonably Practicable) of danger/loss from accidents or incidents T Overall risk can be physical and/or financial T Overall risk = Σ (hazards x associated risks)

4 Risk management 2 - hazards and risk T Hazards are of various types - particular substances (e.g.asbestos, gas) - machines - methods of working - other aspects of work organisation T Risk is the probability that the danger from a particular hazard will be realised

5 Risk management 3 T To calculate the overall risk - the hazard must be identified - the impact of the hazard must be estimated - this involves an understanding of the mechanism relating the hazard and the overall risk T After the calculation, action is taken to remove the hazard or reduce its impact

6 Risk management 4 T In general the relationship or the mechanism connecting the hazard and the overall risk are known, and allow a quantitative risk assessment to be made T In MIC the hazards (i.e. the condition or conditions that give rise to it) are not fully understood, nor are the connections between aspects of MIC and its occurrence

7 A definition of MIC (Microbiallyinfluenced corrosion) T The influence of microorganisms on the kinetics of corrosion processes of metals, mineral and synthetic materials caused by microorganisms adhering to the corroding interface (usually called biofilms) BRITE-EURAM Network on MIC of Industrial Materials BRRT-CT

8 Biofilms: properties related to corrosion T they conduct electricity T new corrosive compounds can be formed within them (e.g. sulphides beneath aerobic biofilms) T they reduce the diffusion of nutrients, biocides and corrosion inhibitors to the metal surface T electrochemical cells can form beneath them, which may cause nucleation of corrosion pits T partial coverage of a surface by a biofilm may lead to under-deposit corrosion

9 Biofilms: some things we don t know T there are no good quantitative chemical or physical models for biofilms T the nutritional requirements for bacteria are not known in detail T the stoichiometries of metabolic reactions are not known in general T many bacteria are unknown or uncharacterised T why some biofilms are protective, and others corrosive

10 Biofilms: some microorganisms that can be involved in MIC T T T T T mainly bacteria, but also fungi, algae aerobic slime-formers, e.g.pseudomonas aeruginosa, can give rise to oxygen concentration cells sulfate-reducing bacteria (SRB), e.g.desulphovibrio vulgaris produce H 2 S from sulphate and thiosulphate sulphur oxidisers, e.g.thiobacillus thiooxidans, produce sulphuric acid by the oxidation of reduced sulphur compounds iron oxidisers, e.g.gallionella, oxidise soluble Fe(II) to Fe(III), precipitating solid FeOOH, beneath which corrosive acidic conditions can form

11 Risk: the essential conditions for MIC T for MIC to occur there must be: - liquid water - a source of nutrients (energy) - a source of carbon, but some bacteria and algae can fix CO 2 - a favourable temperature - a source of microorganisms T if any of these conditions is not met, there is no risk of MIC

12 Risk: where MIC is found again and again T metals in contact with the soil, unprotected against corrosion by coatings, cathodic protection etc. T in stagnant seawater T after hydraulic testing with untreated water T metals in contact with water and liquid fuels or untreated lubricants T comparison with these situations may give a qualitative idea of the risk

13 MIC risk database 1 T since the mechanisms of MIC are very poorly understood, no quantitative model is yet possible T to overcome this, the relationship between different aspects related to MIC and the rates of MIC can be investigated T 32 aspects (in 6 groups) identified and agreed by the participants of a BRITE-EURAM network T values corresponding to these aspects and MIC rates, found in the literature, are placed in a database

14 MIC risk database 2 T some aspects are used to select relevant data T the statistical correlations between each aspect and the corrosion rates are analysed to find the best correlation in each group T a comparison is made between each group to establish the relative influences of the aspects T corrosion rate = Σ (influences x aspects) T use this equation to estimate corrosion rates in new situations of the same type

15 Aspects 1: groups T general T water-related aspects T microbiological data T metabolites T MIC control measures T corrosion-related aspects

16 Aspects 2: general T reference (e.g.85scoa) T material (e.g.316) T facility (e.g.heat exchanger, tube) T environment (e.g.seawater, well water) T temperature ( C)

17 Aspects 3: water T flow conditions (e.g.turbulent, stagnant, intermittent) T nutrients (e.g.yes/no, sulphates, Postgate B) T concentration of nutrients (mg/litre)

18 Aspects 4: microbiological T biofilm coverage (mg/m 2 ) T microorganisms: sessile or planktonic T types of microorganisms (e.g.srb) T were the microorganisms identified? (Y/N) T number of microorganisms (cfu/ml) T technique used for enumeration (e.g.atp)

19 Aspects 5: metabolites T inorganic metabolites (e.g.sulphur, FeS) T concentrations in mg/litre T organic metabolites, including ph T concentrations in mg/litre, or ph value

20 Aspects 6: MIC control measures T type of control used (e.g.biocide, pig) T biocide (e.g. NaOCl) and concentration in mg/litre T treatment duration in hours T interval between treatments in hours T effect on corrosion (Y/N)

21 Aspects 7: corrosion T geometry (e.g.coupon, tube, crevice) T surface (e.g.ground, welded) T corrosion type (e.g.general, pitting) T micrographs provided (Y/N) T corrosion products (colours, compounds) T corrosion rate (mm/yr) T mean or pitting rate T method used (e.g.weight loss, LPR)

22 Experience with the database 1 T most articles and reports on MIC provide no data on corrosion rates T the data on other aspects is often very limited T take care with units (e.g.mm/yr, mpy) T some data may be repeated in different articles T so far, some 500 entries/rows

23 Experience with the database 2 T request for a separate version for stainless steels, for which any MIC considered unacceptable T for stainless steels concentrations of chloride, nitrate and the corrosion potential E are important aspects T the lack of data (ND = no data is the most frequent entry) suggests that the main initial use of the database will be for comparison and not for statistical predictions

24 Experience with the database 3 T data entry made easier using the Data Form facility in Microsoft Excel T entries can be filtered using the Autofilter ou Advanced Filter facility - choose the metal - choose the application - compare with filtered data

25 Conclusions 1: the Future T more data may help to establish relationships between aspects of MIC and MIC T possible use of neural networks to establish relationships T a need for more and better data in reports to define MIC situations better - the 32 aspects T better distinctions between MIC and non-mic

26 Conclusions 2: the Present T risk assessment for MIC is made very difficult because the mechanisms of MIC are inadequately understood T if any of the basic conditions for MIC are not satisfied, there is no risk; if the conditions are satisfied, MIC may happen, or not. We don t yet know why. T MIC is relatively common in a few industrial situations, which can be used for comparisons

27 Conclusions 3: the Present T a database of information on MIC has been contructed, based on scientific articles and reports, which gives data on aspects related to MIC and measured rates of MIC T the database is limited by a lack of qualitative and quantitative information on aspects T the database will probably be most useful to start with for comparisons rather than statistical calculations of risk