Appréhension des risques de biocorrosion dans les installations industrielles

Risk assessment of microbially-infuenced corrosion in industrial plant

Corrosion & Fouling Consultancy, Oxford, Grande-Bretagne

Résumé

L’appréhension des riques est un devoir très important aujourd’hui pour les gérants industriels. En général, on peut réprésenter le dommage comme la somme des produits des dangers (“hazards”) et des risques adjoints. On peut calculer les risques assez facilement si les mécanismes qui lient les dangers et le dommage sont bien connus. Les mécanismes de la biocorrosion ne sont pas bien résolus : la seule présence de micro-oganismes dans une installation industrielle ne fait pas apparaître la corrosion. Pour essayer faire une évaluation du risque, les participants d’un réseau BRITE-EURAM appelé “Biocorrosion” ont créé une base de données de la biocorrosion qui permet en principe de relier statistiquement les aspects généraux, microbiologiques et métallurgiques de la biocorrosion avec les données de la vitesse et la forme de la corrosion avec le dommage en cause. On décrit la construction de la base de données, son usage pour prédire la biocorrosion et faire des comparaisons avec les informations déjà publiées dans la littérature scientifique et technique.

Abstract

Risk management is a vital function for managers in industry today, and is concerned with avoiding or minimising the harm from potential accidents or incidents. The harm, which may be physical or financial, or both, can be represented as a sum of the product of hazards and the associated risks. Hazards can include substances, machines, methods of work and other aspects of work organisation. Risk expresses the likelihood that the harm from a particular hazard is realised. In overall risk assessment both the risk and the severity or impact of the harm must be assessed. Once the assessment has been made, the process of risk management is undertaken to eradicate or minimise the harm from each and all of the identified risks.

The mechanism that links hazards and the harm that they cause is often clear and well-established. However, this is not the case for Microbially Influenced Corrosion (MIC), which has recently been defined by members of a network of European specialists (BRITE-EURAM Network on Microbiologically Influenced Corrosion of Industrial Materials BRRT-CT9S-5084) as «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’). A prerequisite for MIC is the presence of microorganisms. If MIC is based on their activity, (i) water, (ii) energy source, (iii) carbon source, (iv) electron donator, and (v) electron acceptor are required». It is clearly a complex process, which is further complicated by the range of microorganisms, mainly bacteria, that are involved. These include :

(a) slime-formers, which are generally aerobes, and thus can produce electrochemical oxy- gen-concentration cells that cause localised corrosion ;

(b) sulphate-reducing bacteria (SRB), which generate hydrogen sulphide from sulphate or thiosulphate present in the water ;

(c) iron oxidisers, which derive their energy by oxidising iron (II) to iron(Ill) ; the iron (III) hydrolyses, and precipitates as a solid beneath which acidic conditions are generated ;

(d) sulphur oxidisers, which oxidise lower-valency sulphur compounds such as hydrogen sulphide to sulphuric acid ; they can generate pH values below I, which are extremely corrosive to both metals and non-metallic materials such as concrete.

The mechanisms of MIC are relatively poorly understood. The simple presence of microorganisms in a wide range of systems does not generally give rise to corrosion. Other approaches have to be investigated as a means of assessing the risk.

An initial assessment may be made by investigating the availability of the basic requirements for microbial respiration and growth, in particular the presence of water, nutrients, a source of microorganisms and suitable temperatures, as outlined in the definition given above. If these requirements are not satisfied, the risk should be zero. A further qualitative guide to the risk may come from comparing the situation being investigated with industrial situations where MIC has most frequently been reported. These include unprotected metals in contact with soil, unprotected metals in stagnant seawater, corrosion from residual water left after hydrotesting plant, and metals in contact with water and liquid fuels or lubricants.

A more quantitative approach involves setting up a database of MIC data in which quantitative aspects of MIC related to corrosion are correlated statistically with corrosion data in the form appropriate to the harm being considered. This allows the influence, or seriousness of the aspect to be quantified. The overall risk can then be expressed as the sum of the product of influences and aspects. Once the influences have been established for the aspects associated with the harm under consideration, the data can be used to predict the likely harm in new situations.