Upper Stools and Bulkhead Plating
Fig: These holds are unlikely to pass a grain survey, as they are heavily pitted with rust scale and embedded with coal staining
These areas are fully coated and do not have contact with iron ore cargo. The TOW is similar to that for the transverse frames except the critical relative humidity is likely to be lower than 70% due to the presence of coal dust. The hygroscopic nature of coal dust allows an adsorbed layer of water to form at a lower vapour pressure.
Double Bottom Ballast Tank (DBBT)
By IACS rules it is compulsory for the DBBT to be coated. Cathodic protection is also utilised by some operators but is only effective when the tank is ballasted. Coating breakdown occurs on the underside of the tank top plating due to plate deformation caused by grab damage.
The frequency of cargo discharge is therefore a significant operational parameter. After coating breakdown, corrosion proceeds when the tank is unballasted due to the ineffective cathodic protection and high humidity. Deterioration of coating and consumption of anodes is dependent on the time in ballast and the salinity and temperature of the ballast seawater.
This area is a fully coated void space. Similarly to the DBBT, coating breakdown occurs on the underside of the tank top plating. The incidence of coating breakdown is therefore dependent on the frequency of cargo changes. Areas of exposed plating increase with time as the coating debonds from the surface due to differential aeration corrosion around the perimeter of the exposed area. As the duct keel is a void space, corrosion will only proceed when there is moisture present, that is, when the local relative humidity increases above approximately 70%. Corrosion is expected to be approximately linear with respect to the TOW because of the powdery composition of the rust which offers minimal protection and is common on downwards facing surfaces.
Protective coatings and usually cathodic protection is used to prevent corrosion in the topside tanks. As with the DBBT, the deterioration of coating and consumption of anodes is dependent on the time in ballast and the salinity and temperature of the ballast seawater. Additionally the coating on the underdeck longitudinal stiffeners is prone to premature failure, possibly because of increased temperature due to deck heating.
Corrosion at exposed areas is dependent on the time the tanks are not ballasted and therefore cathodic protection is ineffective. However, unlike the DBBT, the humidity is not high and the surfaces dryout with time. The TOW is therefore proportional, but not equal, to the time the tanks are unballasted.
Operational Parameters Affecting Corrosion
The most obvious operational parameter affecting cargo hold corrosion is the relative frequency of transporting coal or iron ore cargoes. Coal is more corrosive than iron ore due to the presence of sulphates and chlorides and its ability to produce an acidic leachate. Furthermore, the TOW and wear of protective coatings varies for different locations within the vessel, depending on which cargo is loaded. Several of the factors contributing to corrosion are dependent on the local environmental conditions the ship encounters.
For example, TOW, sideshell sweating, and degradation of coatings in ballast tanks are dependent on the relative humidity of internal spaces, coal and seawater temperature, and deck heating and seawater temperature respectively. The corrosion rate of each location is therefore dependent on trading routes that determine air and seawater temperature, relative humidity and seawater salinity. Exposed steel in the topside and double bottom ballast tanks corrodes when the tanks are deballasted. Moisture is then present as an adsorbed layer due to the high humidity of the enclosed space. The ballast ratios of each tank are therefore significant.
In summary, it is considered that a corrosion prediction model for bulk carriers should account for the following factors; relative incidence of coal and iron ore cargoes, frequency of cargo loading and ballasting, ballast ratios and expected trading routes.
Corrosivity of coal cargo
Coal is considered to be corrosive to cargo hold surfaces due to its ability to produce an acidic leachate as a result of sideshell sweating (e.g. Jubb, 1995; Coll, 1996). It is desirable to know if the corrosivity of coal is highly variable. If this is true then, in addition to the operating variables identified, it will account for the highly variable corrosion rates reported in the literature for bulk carriers.
To investigate the variability of coal corrosivity it is helpful to consider the fundamentals of why coal is corrosive.
Corrosion proceeds in the presence of moisture and a cathodic reducer, typically oxygen. The inherent moisture in black coals is usually 1-2wt% whilst export contracts specify total coal moisture levels of 7- 10wt%. The 5-9wt% free moisture is adequate to allow corrosion reactions to proceed. The oxygen content within loaded coal is approximately 20.5%, which is similar to open air. Therefore bulk coal will corrode mild steel.
It is proposed that the variability of coal corrosivity is due to;
(a) the supply of free moisture that contains impurities leached from the coal, such as chlorides, sulphates and low pH.
(b) the contact of coal particles at discrete points on the surface causing galvanic and differential aeration effects.
Aggressiveness of Coal Impurities
The most significant impurities in coal are chlorine and pyritic sulphur which provide sources for Cl- and SO4 2- ions respectively. Corrosion is also dependent on the pH of the electrolyte. Corrosion Due to Coal Particle Contact To the authors knowledge, the mechanism of particle contact corrosion (due to bulk cargoes) has not been investigated for ship structures. For coal cargo it is suggested that the rate of steel corrosion at points of contact with coal particles is altered due to two effects; (a) the formation of a galvanic couple between the coal and the mild steel. (b) the presence of a local differential aeration cell at each contact point.
Particulate Galvanic Couple
Coal comprises 50-65% carbon. The reversible potential of carbon is 2.10V compared to -0.44V for mild steel. It is therefore feasible that a galvanic cell will exist.
This is supported by a study of the corrosivity of steelmaking process materials (Barr, 1997). Under identical conditions, mild steel covered with coke corroded at least twice as fast as for other materials such as iron ore, sinter and limestone. This was attributed to a galvanic effect. Analysis of each leachate showed that coke leachate did not have a significantly higher chloride or sulphate concentration.
The corrosivity of coal cargo is highly variable. The supply of chlorine and pyritic sulphur, and leachate acidity contribute to the variability. Bulk carrier corrosion rates are therefore highly variable due to the carriage of different types of coals, in addition to the previously mentioned operating variables.
Age related vessel damage, and hence corrosion, has a significant contribution to bulk carrier losses. Statistical analysis of survey data has revealed that bulk carrier corrosion rates are highly variable. The relative incidence of coal and iron ore cargoes, frequency of cargo loading and ballasting, expected trade routes, ballast ratios and type of coal transported are the main factors contributing to the variability of corrosion rates of bulk carriers.
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