Factors effecting corrosion of ballast, oil tans and cargo holders are also ... are subjected to open air corrosion. Ballast and oil tanks, as well as bulk hulls are.
FACTORS INFLUENCING ATMOSPHERIC CORROSION AND CORROSION IN CLOSED SPACES OF MARINE STEEL STRUCTURES M. Panayotova1, Y. Garbatov2, C. Guedes Soares2 1
University of Mining and Geology, Sofia, Bulgaria 2 Unit of Marine Technology and Engineering Technical University of Lisbon, Instituto Superior Tecnico Av. Rovisco Pais, 1096 Lisboa, Portugal;
Abstract The paper reviews factors, governing marine atmospheric corrosion phenomena on the structural steel component level. Open air and enclosed atmospheric corrosion are considered. Factors effecting corrosion of ballast, oil tans and cargo holders are also briefly reviewed.
1 Introduction The marine structure operates in a complex environment. Seawater properties such as salinity, temperature, oxygen content, pH level and chemistry can vary. There are different types of corrosion that can attack metals and it may be identified with atmospheric rusting of iron base alloys, but that is only one of the many possible mechanisms of corrosive attack. Some types of corrosive attack on metals may be formulated as general corrosion, galvanic cells, under-deposit corrosion, CO2 corrosion, top-of-line corrosion, weld attack, erosion corrosion, corrosion fatigue, pitting corrosion, microbiological corrosion, and stress corrosion cracking Ship constructional steel corrodes easily in marine atmosphere and generally two types of environmental conditions may be distinguished – open and enclosed air. Upper part of ship hull, deck and equipment on deck are subjected to open air corrosion. Ballast and oil tanks, as well as bulk hulls are deteriorated by enclosed atmospheric corrosion. The objective of this work is to review different factors influencing atmospheric corrosion and corrosion in closed spaces of marine steel structures. The key parameters of corrosion are identified and correspondently described.
2 Open Air Marine Atmospheric Corrosion 2.1 Environmental Factors Environmental factors effect the open-air marine atmospheric corrosion of bare steel may be classified as: - Air temperature - highlighted as one of the most important factors [1-4]. Temperature effect is more pronounced at sharp temperature changes [4]. In addition to air temperature, the real temperature on the metal surface (TM) can provide information about the kinetics of the corrosion process [1]. - Time of wetness (TOW) – the time during which the metal surface is covered with thin film of electrolyte. TOW determines the duration of the electrochemical corrosion process and is pointed as very important environmental factor effecting the atmospheric corrosion [1-4]. Increased corrosion rate of carbon steel, found at night and rainy day than in the daytime and sunny day [5], actually represents the influence of TOW. - Air relative humidity (RH) - very important factor [1-6]. The combination of temperature and RH is described as temperature–relative humidity complex (T-RH complex). According to ISO 9223-92, the T-RH complex can be used to calculate the TOW. - Alteration of wet-dry cycles on the metal
surface - accelerates the cathodic reaction of oxygen reduction and correspondingly the total corrosion process [3 and 7]. The influence of solar radiation [8] could be explained in terms of electrolyte evaporation, i.e. wet/dry cycles. - Sea-salt deposition - very important environmental factor [1-5 and 9-11]. Deposited salts decrease the eventual protective properties of corrosion products formed on the metal surface and act as centers of water vapor condensation [3, 4 and 9] that facilitate electrochemical reactions. Chloride ions easily destroy the oxide film formed on carbon steel surface [12]. - Frequency of precipitation [2, 8 and 13] - it controls the pollutants flushing from the steel surface [13]. - Air pollution with gaseous (SO2, H2S, NOx) and solid (industrial smoke, sand particles) pollutants [4, 8 and 11]. - Prevailing wind direction [2 and14]. - Exposure angle - influences morphology of the rust formed on steel surface and hence the corrosion rate [1, 3 and 15]. - Composition, morphology, mechanical (adhesion strength and fracture toughness) and chemical properties of corrosion products - influence significantly the corrosion rate [1, 4, 7, 13 and 15-18]. Rust formed on the surface of constructional steel in chloride media possesses corrosion catalyzing property [18]. Kamimura et. al [17] found that the ratio α/γ* (where α - the mass of α-FeOOH, and γ* is the total mass of γFeOOH, β-FeOOH and Fe3O4 in the rust formed) could be used as an index for evaluating the corrosion resistance of weathering steels in atmosphere with high salt content. Reduction of Fe3+ ions from βFeOOH or γ-FeOOH contributes to cathodic depolarization, in this way increasing the corrosion rate [7 and 18]. - Conditions prevailing during the first month of exposure - influence protective properties of the corrosion products and corrosion rate [4, 19 and 20]. A conclusion is drawn that the environmental factors control the corrosion rate in the initial exposure period (during the first year) and the adsorption of anions on the corrosion product films controls the corrosion in the latter period [11].
Usually authors grouped several factors acting together (as independent) and affecting the corrosion rate. Many authors consider TOW, chlorides deposition (or Clconcentration in the air), air RH and temperature [1, 2, 9 and 10]. Phull and Pikul [2], based on 38-years atmospheric corrosion monitoring, added prevailing wind direction and rainfall to TOW, Cl- deposition, air temperature and RH as factors jointly effecting the corrosion rate. It has to be pointed out that the factors highlighted in the literature as affecting the rate of atmospheric corrosion of constructional steel not only act together, but they are interrelated and interdependent. Air temperature and relative humidity determine the TOW [1]. Air temperature influences diffusion and solubility of oxygen in electrolyte layer on the metal surface and directly affects the corrosion rate in the very initial (kinetic stage). T-RH complex, TM, and the presence on the surface of hygroscopic pollutants or corrosion products (such as chlorides [1, 9 and 12] and sulfates [9 and 13]), affect the formation of thin electrolyte layer on the metal surface, its thickness and life. Hygroscopic species decrease the critical RH value, i. e. the value above which the corrosion rate increases sharply due to the initiation of water vapor capillary condensation on the metal surface. The frequency of precipitation, as well as exposure angle, directly influences TOW and the thickness of electrolyte layer. Solar radiation and wind direction affect the TOW and the thickness of electrolyte layer. The exposure angle affects the temperature of the metal surface, metal absorbance, emissivity and thermal conductivity, and morphology of corrosion products. In this way it influences the possibility for thin electrolyte layer formation on the metal surface. So, it may be stated that TOW and thickness of electrolyte layer on the metal surface are integral parameters, which count for climatic factors (temperature, RH, frequency of precipitation, wind direction, duration and intensity of solar radiation), salts deposition and exposure angle. That is why one should agree with the idea raised by Veleva and Alpuche [1] that it is better to measure the TOW directly on the corroding metal surface,
than to calculate it (by using the T-RH complex). The time of initial exposure could be assumed as an integral environmental parameter, accounting for the climatic factors (temperature, RH, frequency of precipitation, wind direction, duration and intensity of solar radiation). Concentration of chloride salts on the metal surface affects TOW, mainly determines the corrosion controlling reaction and is decisive for the nature and properties of corrosion products formed on the metal surface. General corrosion is mainly observed in the case of marine atmospheric corrosion of bare structural steel. Average corrosion depth can be used as main parameter. The real corrosion affect (mass or thickness loss) due to atmospheric corrosion represents a result of electrochemical corrosion, i.e. operation of corrosion cells [3, 4, 19 and 21]. The corrosion cells operate only when electrolyte is available on the corroding metal surface. 2.2 Open Air Corrosion Factors affecting the formation and thickness of electrolyte layer on the metal surface would influence the corrosion rate. The electrolyte can present as very thin solution layer invisible by naked eye, visible by naked eye solution layer, and layer of hygroscopic and wet corrosion products [4, 6 and 19]. Kinetics of anodic and cathodic reactions is determined by the thickness of electrolyte layer and not by the time when the surface is kept visibly wet [7]. Oxygen diffusion to the metal surface is facilitated in thin surface layer of electrolyte, compared to bulk electrolyte. This effect increases with decreasing the electrolyte layer thickness. Facilitated O2 transport principally results in two main effects: acceleration of the cathodic reaction of oxygen reduction and facilitation of the anodic passivation. In the case of marine atmosphere, availability of Cland hygroscopic salts prevents passivation and anodic concentration polarization. Marine salts are dissolved in electrolyte layer on the metal surface ensuring its enough high conductivity and eliminating the possibility for ohmic control. Consequently, it could be assumed that the atmospheric corrosion in marine environment
proceeds mainly under cathodic control. The major material loss would occur under enough thick electrolyte layers on the metal surface [3, 6, 7, 22 and 23]. Factors influencing the cathodic reaction would be the main factors affecting the total corrosion rate of steel in marine atmosphere. Oxygen reduction O2 + 2H2O + 4e- → 4OH-
(1)
is the main cathodic depolarizing reaction, even if the solution layer on the metal surface is slightly acidic [4]. It has been established that the corrosion with oxygen depolarization proceeds mainly under kinetic control only when very thin electrolyte layer is available on the metal surface [6]. For very thin electrolyte layers (less than 10 µm [7] or 20 µm [6]) the current of O2 reduction (it could be expected also the corrosion rate) does not depend on the electrolyte layer thickness. When relatively thick electrolyte layer, or layer of wet corrosion products present on the metal surface, the reaction rate depends on the O2 transportation to the metal surface through electrolyte layer and layer of corrosion products. It has been found experimentally [7, 14 and 36] that the maximum rate of O2 reduction reaction is observed under electrolyte layers where the O2 reaches the surface mainly by diffusion through the electrolyte. The maximum corrosion current icorr (consequently, the maximum corrosion rate) can be estimated using the Nernst-Fick equation [19]: icorr. = iO2 = 4 x F x D x CO2 / δ
(2)
where iO2 – cathodic current density at entire O2 consumption on the metal surface, D – oxygen diffusion coefficient, CO2 – concentration of dissolved O2, F – the Faraday’s constant, δ - thickness of the electrolyte layer and 4 – number of electrons assimilated by one oxygen molecule. According to some authors [3, 6, 22 and 23] the oxygen reduction rate is the highest at layer thickness of 20-70 µm, and then decreases sharply up to layer thickness of 150 – 200 µm, followed by a very slow linear decrease up to electrolyte layer thickness in
the range of 500 – 700 µm. Corrosion rate remains nearly constant at electrolyte thickness over 600-700 µm. Oxygen diffusion coefficient (in a liquid) increases with the temperature:
following depolarization reactions [4, 24]:
D = Do x T x exp [-ED / (R x T)]
Based on the above literature survey and discussion, it could be summarized that TOW (measured on the metal surface), thickness of electrolyte layer on the metal surface, temperature and chlorides deposition on the metal surface are the factors which influence significantly the atmospheric corrosion of bare constructional steel in marine environment. Season of initial exposure and concentration of air pollutants (different from sea-born salts) are to be considered in expert estimation of atmospheric corrosion in marine environment. This estimation could use the ratio α-FeOOH / (γ- FeOOH + β-FeOOH + Fe3O4) in corrosion products as index of steel corrosion resistance. Preferential weld corrosion in marine atmosphere is not expected, if factors leading to base metal ennoblement are considered. Deposition of solid particles (sand, salts) on the surface of coated structural steel would facilitate coating breakdown. Alteration of wet/dry cycles on the metal surface is more influential factor - compared to the case of marine atmospheric corrosion of bare steel. For painted steel with damaged coating corrosion would depend on the chemical composition of the coating (in addition to the above-mentioned factors influencing the open-air marine corrosion of bare steel). At this case the ratio α-FeOOH / (γ- FeOOH + β-FeOOH + Fe3O4) in corrosion products is not expected to be a reliable index of steel corrosion resistance.
(3)
where Do is constant, ED is activation energy of diffusion, T is temperature and R is gas constant. Concentration of dissolved oxygen in electrolyte layer depends on electrolyte temperature and dissolved salt concentration. For the range of temperatures and salinity characteristic for the marine environment, the concentration of dissolved oxygen slightly decreases with temperature and concentration of dissolved salts. This effect is offset by increased O2 diffusion rate and as a result - an increased corrosion rate of unalloyed steel is usually observed with increasing the temperature up to 30-35 oC [22]. When corrosion products are already available on the metal surface, their adhesion and density affects the diffusion of O2 and water to the metal surface. In presence of high air-born salinity, corrosion product layer possesses higher porosity and reduced resistance to O2 diffusion [8, 12]. The porosity increases and resistance to O2 diffusion decreases with increasing the share of β-FeOOH in corrosion products. Additional (to the O2 reduction) cathodic depolarization reaction of reduction of Fe3+ ions from β-FeOOH [4 and 18] contributes to the increased atmospheric corrosion rate in marine environment, compared to clean air. The amount of β-FeOOH in the rust layer increases with increasing the Clconcentration and time [11 and 18]. Consequently, the chlorides precipitation and concentration in the atmosphere increases the corrosion rate through the influence on TOW, O2 transportation to the surface, corrosion products and by direct influence on electrode reactions. Presence in the atmosphere of pollutants, which are able to act as cathodic depolarizers in addition to O2, facilitates the cathodic reaction and increases the corrosion. Such pollutant is SO2, which in wet environment forms H2SO3 and this way participates in the
2H2SO3 + H+ +2e- → HS2O4- + 2H2O
(4)
HSO3- + 5H+ +4e- →So + 3H2O
(5).
3 Enclosed Marine Atmospheric Corrosion 3.1 Environmental Factors The most effecting factors to corrosion have been classified as: - TOW, salt deposition and temperature – found as main factors effecting atmospheric corrosion of empty ballast and cargo tanks [8 and 25].
- Sulfur dioxide - in concentration of 250 ppm in the inert gas environment of crude oil tankers increases severely the corrosion of steel [24] because it acts as additional cathodic depolarizer. The corrosion rate increases with increasing the temperature and O2 and SO2 content in the inert gas [26]. - Oil concentration in water layer left on steel surface inside washed and unloaded oil tank [27] - increased oil concentration facilitates the cathodic reaction of oxygen reduction due to the enhanced solubility of O2 in oil compared to water, while inhibiting the anodic reaction of metal dissolution. It can be stated that the atmospheric corrosion in enclosed ship spaces proceeds under cathodic control and O2 reduction is the main cathodic depolarization reaction. General corrosion is mainly observed. Average corrosion dept can be used as main parameter to describe the corrosion. Factors, which effect the enclosed atmosphere corrosion in sea-ships, are TOW, thickness of electrolyte layer on the metal surface, temperature and chloride deposition. Concentration of residual oil and detergents in surface electrolyte layer and small amounts of SO2 in atmosphere would increase the rate of enclosed atmosphere corrosion in sea-ships. 3.2 Enclosed Spaces Corrosion Ballast tanks are subjected to enclosed atmospheric corrosion in loaded condition and to corrosion in contact with seawater in ballasted condition. Corrosion of ballast tanks full with seawater is influenced by the factors effecting the corrosion of constructional steel immersed in seawater in its initial stages (kinetic and oxygen diffusion controlled). A significant influence of MIC on the total corrosion rate is not expected due to the relatively short time when the ballast tank is full with water. Water temperature (that inside ballast tanks is normally warmer than “at-sea’ condition [28]) is very important factor influencing the corrosion. Concentration of dissolved O2 in ballasting seawater and properties of corrosion products (their adhesion, density, porosity and ability to participate in Fe3+ reduction reactions) formed on the inner surface of the tank, are recommended for consideration in an expert assessment of total
corrosion of ballast tanks. Frequency of ballasting-deballasting cycles would influence the corrosion process. Accelerated corrosion is expected at higher frequency, due to alternation of dry and wet conditions and formation of corrosion products able to act as cathodic depolarizers. When the inner surface of the ballast tank is coated, the corrosion development will also depend on the coating properties [28] and water composition and salinity. Higher frequency of alteration of ballastingdeballasting conditions is expected to lead to faster coating degradation. Coating degradation would lead to increased corrosion in deballasted condition when the inner tank surface is painted and cathodically protected. Increased coating degradation and anodes consumption is expected in ballasted condition. The increase will depend on the time in ballast, water composition, salinity and temperature [29]. Oil tanks are subjected to enclosed atmospheric corrosion when are empty and to corrosion caused by the contact with crude oil when are full. Corrosivity of crude oil and oil products is due to their impurities (sulphur, hydrogen sulphide, thiols). In this case the metal destruction is caused by heterogeneous chemical reactions. The corrosion rate is influenced mainly by the pollutant (actually oxidant) concentration near the metal surface. This corrosion effect is much lower, compared to the effect of electrochemical corrosion observed when water (even in trace concentration) is available in the oil. The corrosion rate is further increased when salts present in the liquid. In our opinion, the corrosivity of crude oil polluted with water and salts has to be considered in an expert assessment of the total corrosion of oil tanks, nevertheless that it is considerably lower compared to enclosed atmospheric corrosion. Cargo holds are subjected to enclosed atmospheric corrosion when are empty and to corrosion caused by steel contact with the cargo or cargo leachate [8 and 30-31]. Concentration of Cl-, SO42-, H+ and HCO3ions in leachate represents a factor
influencing the corrosion of cargo holders, besides the temperature and dissolved O2 content. Composition, moisture content and particle size of the load, as well as thickness of layer of loaded material on steel surface are important factors effecting the corrosion of cargo holder in loaded condition. Frequency of cargo loading and ballasting influences both corrosion development and longetivity of an eventual steel coating.
4 Conclusion Temperature on the metal surface, TOW, electrolyte layer thickness and Cl- deposition on the metal surface are the main factors affecting the open air and enclosed atmospheric corrosion of bare structural steel. Chemical composition of the coat has to be added to above-mentioned factors for painted surfaces. Season of initial exposure (for open air) and residual oil and detergents concentration in the surface electrolyte layer (for enclosed atmosphere) have to be considered. When evaluating the corrosion of enclosed spaces (ballast, oil tanks and cargo holders) both enclosed atmospheric corrosion and corrosion due to steel contact with carried material is to be taken into account.
5 Acknowledgement This paper has been developed in the project ”Safety and Reliability of Industrial products, Systems and Structures (SAFERELNET)”, which is financed by the European Union through the Program “Competitive and Sustainable Growth”, under Contract No. G1RT-CT-2001-05051.
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