PREVENTION OF DAMAGES BY CORRECT SELECTION OF MATERIALS

Transcription

1 PREVENTION OF DAMAGES BY CORRECT SELECTION OF MATERIALS Margit Weltschev Federal Institute for Materials Research and Testing, Berlin, Germany; Compatibility evaluation data for metallic and polymeric materials under the influence of chemicals have been collected and maintained on the base of laboratory tests, operational experiences and literature data in the Federal Institute for Material Research and Testing (BAM) in Germany since 20 years. Compatibility evaluations of metallic tank materials and polymeric gasket, coating and lining materials are part of the substance-related prototype approvals for tank containers and portable tanks by the BAM. These data also serve as source of knowledge for compatibility data for railway tank cars and tank vehicles (figure 1). However compatibility evaluations are not only relevant for the safe transport of dangerous goods and water-polluting substances but also for tank and plant construction. The number of inquiries for the selection of resistant materials for transport containers, storage tanks, pumps, pipelines etc. has been increased in the last years. The BAM tries to satisfy the widespread need for information at least partly by publishing these compatibility data. The variety of corrosion types which can occur at construction materials cause different types of losses with quite varying risk potentials. Uniform corrosion is easily to recognise and technically detectable. However crevices, pitting, internal material separations and embrittlement occurrences can occur unforeseeably and are far more dangerous thereby. The caused leakages, breakages and crack occurrences are damage types that may represent a high endangerment for humans and environment. Depending on the energy releasing rate toxic, combustible or explosive substances can leak. Crevice corrosion ranks among the most dangerous ones that are often recognized for the first time when the container becomes leaky. Protection against corrosion already begins with the selection of a corrosion resistant material for handling a certain medium. The term corrosion is common for metals, for polymers terms such as degradation, dissolution, resistance or chemical stability are used. Users who are not corrosion or polymer scientists can use the compatibility data lists of the BAM to answer the question: Which material is suitable for a special medium? However the application of the compatibility data requires the consideration of specific operational conditions. The publication of the compatibility data by the BAM should contribute to prevent corrosion damages by correct selection of materials. KEYWORDS: dangerous goods, transport, storage, tanks, resistance, metallic materials, polymeric materials CORROSION RESISTANCE OF METALLIC MATERIALS Corrosion resistance is not a material property but a material behaviour determined by interactions between the material surface and the ambient medium. Both, medium and material affect the corrosion resistance. In EN ISO 8044 Corrosion of Metals and Alloys Basic Principles and Definitions ( ) corrosion is defined as an interaction between a metal and its surrounding. A material is corrosion resistant if this interaction does not result in corrosion occurrences or in such ones that can be regarded as tolerable. Corrosion resistance is thus the ability of a metal to keep up the functional capability in a given corrosion system. If the corrosion resistance of a metallic material has to be evaluated, the ambient medium and other important conditions for the possible reaction, such as surface condition, temperature and flow rate have to be included into the evaluation. Standardised corrosion tests are to be interpreted and evaluated under consideration of these substance-related operational conditions as well. They reflect the interaction of the medium under test conditions and may serve as guidebook for practical use. In particular cases a corrosion test under the specific operating conditions has to be performed. Corrosion reactions at container materials lead to measurable material modifications and to corrosion occurrences like. formation of corrosion products,. uniform corrosion and corrosion cracks. This metal modifying corrosion occurrences are based on certain types of corrosion. The variety of corrosion types which can occur at construction materials cause different types of losses with varying risk potentials. Uniform corrosion is easily to recognise and technically detectable. Crevices, pitting, 1

2 Austenitic stainless CrNi- and CrNiMo-steels The most common and frequently used austenitic stainless steels (see table 2) belong to two types of alloys with a basic compositions of 18% chromium and 8% nickel (CrNi-steel) or 17% chromium, 12% nickel and 2 3% molybdenum (CrNiMo-steel) respectively. There are various materials within the two groups of steels which differ in the content of carbon (e.g and , and ) or are characterised by a stabilisation with titanium or niobium (e.g , ). Some of them are alloyed with nitrogen (e.g , ). Figure 1. Tank vehicle produced of alloy 31 (1.4562) intergranular corrosion and embrittlement can occur unforeseeably and are far more dangerous thereby. The caused leakages, breakages and cracks are damage types that can represent a high endangerment for humans and environment. Depending on the energy releasing rate, toxic, combustible or explosive substances can leak. Crevice corrosion ranks among the most dangerous ones that are often recognized for the first time when the container becomes leaky. Protection against corrosion already begins with the selection of a corrosion resistant material for handling a certain medium. The evaluations specified in the BAM-List Requirements for tanks for the carriage of dangerous goods have originally been provided for tank containers and portable tanks for the carriage of dangerous goods [1]. These data are also transferable to other practical applications. Considering the respective practical conditions they are applicable as guidebook for packagings, IBC s, storage tanks, pipelines etc. made of metallic materials. OVERVIEW OF METALLIC MATERIALS The following frequently used metallic materials for tank construction are included into the evaluation. Unalloyed steels In table 1 are listed commonly used unalloyed steels, for which compatibility evaluations of the substance list can be applied under consideration of the specific conditions. Table 1. Unalloyed steels of compatibility evaluations Steel grade Material-no. Standard S235JR EN S235J EN S235J2 þ N þ N EN S275J2 þ N þ N EN P235GH EN P265GH EN P295GH EN Special austenitic higher-alloyed stainless steels The so-called superaustenitic steels with high contents of chromium (20 25%), molybdenum (4 7%) and nitrogen ( %) have been developed for applications with corrosive dangerous goods. These materials need nickel contents of approx % to maintain their austenitic structure. By developing these steels it has been considered that chromium, molybdenum and nitrogen increase the resistance to pitting and crevice corrosion. The corrosion resistance of the two superaustenitic steels alloy 926 (1.4529) and alloy 31 (1.4562) to corrosive dangerous goods was tested in a comprehensive test programme within a research project. Nickel-Chromium-Molybdenum alloys Nickel-chromium-molybdenum alloys are also called C- alloys. Within these alloys the material shows the optimal combination of multifunctional corrosion resistance and good workability. No further elements such as tungsten and iron are contained besides the alloying components nickel (59%), chromium (23%) and molybdenum (16%). This material shows a very good resistance to mineral acids, organic acids and mixed acids, even if they are contaminated. The corrosion resistance of this C-alloy to corrosive dangerous goods was investigated in a comprehensive test program of a research project. Aluminium and aluminium alloys The compatibility evaluations for aluminium with at least 99.5% aluminium (EN AW-1060A) are also applicable to aluminium alloys with improved strength properties, e.g. alloy AlMg4.5Mn (EN Aw-5083). This alloy is often used in tank construction. These alloys belong to the group of sea water-suitable aluminium kneading materials because of their large corrosion resistance. Zinc (layer metal for the production of galvanized steel tanks). Zinc is a non-noble metal and dissolves actively during electrolytic corrosion. The formation of protecting surface layers can inhibit this dissolution, which results in the good corrosion resistance of zinc in a lot of chemicals. Zinc is used as layer metal for the production of galvanized steel containers and tanks by hot-dip galvanisation process. 2

3 Table 2. Standard austenitic stainless steels of the compatibility evaluations Chemical composition: [Weight-%] Material-no. DIN-identification C Cr Ni Mo Cu N other X2CrNi X6CrNi Ti X5CrNiMo X2CrNiMo X2CrNiMoN X6CrNiMoTi Ti FUNDAMENTAL DATA Compatibility evaluations are based on:. literature data,. operational experiences and. laboratory tests Numerous data, which were determined in laboratory tests or operational experiments, are available in form of tables or diagrams. It has to be considered that literature data are based on laboratory tests which are not representative in each case. The knowledge of the test conditions of literature data and corrosion tests is very important for the evaluation of the corrosion resistance. Even small changes in the operating conditions have an effect on the corrosion values. GENERAL PRINCIPLES OF EVALUATION The compatibility evaluations are based on the following evaluation principles The compatibility evaluations do not only evaluate the corrosion, which is caused by a substance contacting the shell material, but also possible dangerous effects of the shell material onto the substance, such as a catalytic decomposition of chemically unstable substances (e.g. hydrogen peroxide). Only those reactions are taken into consideration, which are relevant for a safe handling of dangerous goods. Other factors of practical relevance in material selection, like a deterioration of the medium quality by the shell material, are not subject of the compatibility evaluations. Operating temperature Unless otherwise specified, the compatibility evaluations apply for mean working temperatures up to a maximum of 508C. Purity of substances Even if no special conditions are required for the purity of substances charged into the tanks, the compatibility evaluations apply only for commercially pure (industrial) substances. They do not apply to waste or mixtures with an uncertain number and concentration of additions or impurities in any case. The corrosion resistance of waste or mixtures has to be evaluated on the basis of suitable tests. It is assumed that tanks including their fittings are constructed at least according to the state-of-the-art technology. This applies in particular to welds and the conditioning of the inner tank shell. The following general rule can be applied when selecting a welding filler metal for lowalloyed steels: low-alloyed steels are welded with similar filler metals, while high-alloyed steels are welded with dissimilar (e.g. austenitic) filler metals. The filler metals for welding high-alloyed steels have to meet the requirements specified in the respective material standards. Consideration of test intervals for the compatibility evaluation A compulsory internal visual inspection of transport tanks is required after a certain period in the dangerous goods regulations. For tank containers, this test interval is generally five years in accordance with ADR/RID [2, 3], cap. 6.8, and in accordance with IMDG Code [4] years, as for road tank vehicles is required a test interval of six years. For rail tank cars the required test intervals are eight years for the inner inspection in the dangerous goods regulations RID. The principle of the consideration of test intervals as a standard for the compatibility evaluations of containers for the transport of dangerous goods can be transferred to other fields, such as the storage of chemicals, too. CRITERIA FOR COMPATIBILITY EVALUATIONS Suitability of material-substance-combinations with longterm test intervals (e.g. 5/6 years) Material-substance-combinations are evaluated for test intervals of five or six years as suitable if. uniform corrosion causes a reduction of the shell thickness of less than 0.1 mm per annum and. localized corrosion in form of pitting corrosion, stress corrosion cracking and crevice corrosion are to be excluded. Suitability of material-substance-combinations with shortterm test intervals (e.g. 2.5/3 years) Material-substance-combinations are evaluated for shortterm test intervals of or three years as suitable if. uniform corrosion causes a reduction of the shell thickness of less than 0.5 mm per year and 3

4 . localized corrosion occurrences in form of pitting corrosion, stress corrosion cracking and crevice corrosion are to be excluded. Unsuitable material-substance-combinations Material-substance-combinations are evaluated as unsuitable if. material loss caused by uniform surface corrosion does exceed 0.5 mm per year,. the substance causes stress corrosion in a temperature range necessary under operating conditions,. other localized corrosion occurrences such as pitting corrosion are to be expected systematically independent of the conditions to be complied with,. the substance reacts dangerously with the tank shell in any other way (e.g. catalytic decomposition of the substance). Each of the listed criteria itself excludes suitability. It has to be remarked that a - doesn t mean in every case that a material is not suitable for a special substance in every case. Since the substance list is a public list, the suitability of special material-medium-combinations was evaluated with -, according to the conservative principle, to prevent cases of damage, which could result from inappropriate handling. The tank shell materials can be used by means of shorter test intervals (e.g. six months, one year). INDEX OF CONDITIONS If a positive material is ensured only under certain conditions, then these conditions are specified in coded form in the section conditions. Examples for substance-related and operational conditions: A: Anhydrous A1: Water content less than 10% B: Bromide- and chloride-free C: Free from acids (ph-value 6.5 to 8.5) H: Liquid temperature at the tank shell less than 308C I: Corrosion inhibitors are required, e.g. amines or ammonia N: Tanks/containers are to be pressurised with 0.5 bar (overpressure) of nitrogen or another suitable dry gas. COMPATIBILITY EVALUATION OF POLYMERIC GASKET, COATING AND LINING MATERIALS The resistance of polymer materials to chemical exposure depends on the chemical composition and the structure of the polymeric material (chemical composition of the polymer, fillers and additives), the composition of the interacting medium as well as the exposure conditions. Depending on the kind of exposure, the interacting medium can be associated schematically to one of the following two groups:. Physically active media, which do not react with the polymer but cause swelling up to dissolution and reversible changes of polymer properties.. Chemically active media, which do react with the polymer and cause irreversible changes of polymer properties. It is characteristically for the chemical structure of polymers that even negligible chemical changes can cause very distinct changes in their physical properties. CHEMICAL EXPOSURE WITH SIMULTANEOUS MECHANICAL STRESS Simultaneous mechanical stress is an important factor for the behaviour of polymers in aggressive media. Mechanical stresses themselves can degrade covalent bonds in the macromolecule but they can also activate the molecule. At most practical applications the plastic materials and elastomers are stressed chemically and mechanically at the same time. Therefore chemical relaxation, creep strength and behaviour during stress cracking have to be investigated. OVERVIEW OF POLYMERIC MATERIALS The following polymeric materials which are frequently used in tank construction were included into the evaluation whereas the abbreviated terms for these materials are defined in the EN ISO (2002): Plastics symbols and abbreviated terms Part 1: polymers and their special characteristics.. Polytetrafluoroethylene (PTFE). Vinylidene-fluoride-hexafluoropropylene copolymer (FPM). Perfluoro-alkoxyl copolymer (PFA). Ethylene-chloro-trifluoro-ethylene copolymer (E-CTFE). Polyvinylidene-fluoride (PVDF). Acrylonitrile-butadiene rubber (NBR). Hydrated acrylonitrile-butadiene rubber (HNBR). Natural rubber (NR). Isoprene rubber (IR). Butyl rubber (IIR). Ethylene-propylene-diene rubber (EPDM). Chloroprene rubber (CR). Chlorosulfonated polyethylene (CSM). Styrene-butadiene rubber (SBR). Acrylate rubber (ACM). Methyl-vinyl-silicone rubber (MVQ). Methyl-fluoro-silicone rubber (MFQ). Polyester-urethane rubber (AU). Polyethylene (PE). Polypropylene (PP). Polyvinyl chloride (PVC). Polyamides (PA) 4

5 FUNDAMENTAL DATA Compatibility evaluations are based on the following documents:. literature data,. operational experiences and. laboratory tests The resistance to chemical active media can be estimated by simple immersion tests in the respective medium with approximately the same or intensified exposure conditions (increase of temperature, pressure or flow rate). Time- and equipment-consuming test methods to evaluate the permeation, the absorption as well as the resistance to stress cracking are required in many cases. Apart from the visual evaluation of detectable changes, changes in weight and dimension as well as the course of mechanical and other physical characteristics, depending on the immersion time are the parameters of immersion tests. They are consulted as classification characteristics. In literature and company leaflets frequently used classifications are resistant, conditionally resistant and not resistant. A reference for the compatibility evaluation of elastomeric materials to liquid media is the international standard ISO 1817: Elastomers determination of their behaviour to liquids. This standard describes procedures for the determination of the resistance of vulcanised elastomers to liquids, whereas the change in mass, volumes, dimensions, hardness and tensile stress strain characteristics before and after the exposure to test liquids are measured. The compatibility data of the following material list are suitable for a screening of materials. However, the evaluation of a practice-relevant chemical resistance should always include the mechanical stress too. The creep strength which is the behaviour under long-term mechanical stress is a characteristic value for the practical use of polymeric materials, because in contrast to most metallic materials, polymers tend to shift strongly in their molecular structure already at room temperature. This creep behaviour, as a time- and temperature-dependent procedure, can be strongly affected by the simultaneous exposure to chemicals. GENERAL PRINCIPLES OF EVALUATION In contrast to the compatibility evaluation of metallic materials, the compatibility evaluation of polymeric materials includes only the effects of the charge onto the gasket, coating and lining material. The evaluation of the polymeric materials was carried out in a very conservative way, similar to the metallic materials. The regarded materials can show large ranges in their material properties, like varying cross-linking degrees, different fillers and plasticisers. For the evaluation only literature data and resistance tables of the raw material manufacturers and operational experiences of the users for the reproduction of their compatibility statements were considered. A gasket and lining material was evaluated as resistant, if the resistance was proved up to a temperature of 608C. REFERENCES 1. BAM-Liste Anforderungen an Tanks für die Beförderung gefährlicher Güter, 8. Auflage 2005, BAM, Fachgruppe III.2, Berlin. 2. Accord européen relatif au transport international des marchandises Dangereuses par Route. 3. Règlement concernant le transport International ferroviaire des marchandises Dangereuses par Chemin de fer. 4. International Maritime Dangerous Goods Code (Internationaler Code für die Beförderung gefährlicher Güter mit Seeschiffen). 5