Ballast Tank Weld Repair - Assignment Example

Preferred for its low cost, high mechanical strength, ease of availability and fabrication, constructional/structural carbon steel however gets corroded easily in seawater. Spontaneous oxidation of the metal at its interface with a hostile marine environment causes it to rapidly lose material and thereby its strength, which leads to structural failure, unless adequately protected. The USCG inspection case of thinning of material caused by corrosion and pitting on the weld joint of a ballast tank is one such typical but routine marine engineering event taken up for examination, analysis and repair. Chemistry of Corrosion It is essential to understand the simple chemistry of corrosion to combat it effectively. Mild steel by nature offers poor resistance to corrosion because of its inherent heterogeneous impurities, which facilitate emergence of anodic and cathodic nodes on the surface of the steel. Oxygen and water react with these nodes vigorously causing the metal to lose its electrons to form metallic ions, represented by the anodic reaction: 4 Fe ? 4 Fe++ + 8 e? Iron Ferrous Electrons Atom Ion The above electrochemical corrosion process is feasible only if there is a suitable electron acceptor to combine with the electrons released by the iron atom. Seawater containing dissolved atmospheric oxygen readily serves this purpose. The oxygen is electrochemically reduced to hydroxyl ions in the following cathodic reaction: 2 O2 + 4 H2O + 8 e? ? 8 OH? Oxygen Water Electrons Hydroxyl Ions The heterogeneous character of steel allows for some of its sites to favour the anodic reaction and for others to the cathodic reaction. The ferrous ions and hydroxyl ions formed combine together to produce ferrous hydroxide: 4 Fe++ + 8 OH? ? 4Fe (OH) 2 Ferrous Hydroxide The ferrous hydroxide formed reacts with more oxygen to form hydrated ferric oxide, the familiar reddish brown rust – the telltale symptom of corrosion. 4 Fe (OH) 2 + O2 ? 2 Fe2O3H2O Rust The complex chemical reactions cited above in effect take place concurrently with a host of other factors coming into play and are represented by the single summary equation: 4 Fe + 3 O2 + 2 H2O 2 Fe2O3.H2O Iron Oxygen Water Rust The principal factors which govern the rate of seawater corrosion are oxygen concentration, water temperature, pH and the presence of metals in the form of dissolved salts such as oxides, chlorides, carbonates, sulphates and sulphites in the mineral state of their stable oxidised condition. In view of the large amount of energy expended for the extraction of a metal by the reduction process, there is a sustained pressure on the metal to revert back to its stable low energy oxidised state in the given environmental conditions. The driving force for corrosion is the energy differential between the pure metal and its oxidised forms in the scenario of ever varying ambient conditions. Pitting Corrosion Penetrative localized attack resulting in formation of deep crevices causing thinning of the parent steel material and revealed by rust is characteristic of Pitting Corrosion. It takes place more often in submerged bottoms, inaccessible edges and corners, and at high temperature locations like in the hull of ships where diffusion is easy. The locations most susceptible to corrosion in water ballast tanks are: upper surface of face plate of bottom longitudinals, bottom girders and bottom transverses; upper surface of shell and bulkhead longitudinals; Cut edges of slots and lightening holes in horizontal girders; upper surface of horizontal stiffeners and brackets; deck longitudinals; upper part of deck transverses; and, upper part of longitudinal and transverse bulkheads. If left unattended, rust can grow at the rate of 0.22 mm/year, though the assessed average rate is 0.005 mm/year. ASTM D 610 Standard, supported by ISO 4628: 2003 Standard provides further insight into the mechanics and assessment of the degree of rusting. Corrosion Prevention The popular prevention techniques in modern use have been logically derived from the manifestations of the symptoms of corrosion. In simple terms, they consist of suppression or elimination of the chemical reactions described earlier by cathodic protection and coatings, often used in conjunction for better results. The electrochemical concept of metal with a lower potential suffering higher corrosion when two dissimilar metals are exposed to seawater comes to play in the choice of metals like zinc, aluminium etc. along with mild steel in the marine industry. The natural affinity of a metal to revert back to its stable state is gainfully exploited by the usage of ‘sacrificial anodes’ for effecting ‘cathodic protection’. By virtue of their higher rate of corrosion, zinc or aluminium become the sacrificial anodes, thereby offering protection to the mild steel cathode. Galvanising of steel using a thin layer of metallic zinc is yet another example of cathodic protection. However, before embarking upon ‘coating’ as a solution to corrosion prevention, preparations to be made for weld-repairing including the choice of appropriate non-hazardous materials and the safety precautions to be undertaken, commensurate with the location-specific demands of the work site need to be dealt with. Advance Preparations for Repair Welding The basic recommended work practices for ‘Surface Preparation’ in advance of a ‘weld-failed’ site with specific reference to ballast tanks are given hereunder, sequentially: Sharp and gas-cut edges must be removed by grinder or disc sander; rolled steel sections with radiused edges however can be left untouched. Weld spatter must be removed before grit blasting with chipping hammer or grinder; grinder or disc sander can be used for removing hard spatter. Laminations, if any, should be removed with grinder. Undercuts exceeding classification ruling should be repaired by welding and grinding. Sharp profiles of manually deposited weld beads should be grinder-smoothened. The desired level of surface roughness should be achieved by grit blasting. ISO 8501, 8502 and 8503 Standards in conjunction with ISO 12944 provide further guidance for preparation of steel substrate. It involves assessment of surface cleanliness, surface roughness characteristics of blast cleaned substrate and performing the conductivity test, for final measurement of surface cleanliness keeping in view the planned target useful life after repair, usually ranging from 5 to 15 years. Coating Terminologies ‘Coating’ often synonymous with painting, involves spraying on the metal surface to deposit a protective film, 0.2 - 0.5 mm thick. A common binder used in paints or coatings for marine applications is called epoxy. Coal tar as a binder in epoxy acts as a pigment and influences the flexibility and water resistance of the cured coating film. The overall thickness of a single or multiple coating of layers is called ‘film thickness’ and for fully cured coatings, the term ‘Dry Film Thickness’ (DFT) is used. ISO 2808 deals with the quality control test meant for checking and measurement of DFT. Metal surfaces when cleaned to varying intensities of blasting, the corresponding surface appearances which emerge have been categorised as Sa 1 to Sa 3 as per ISO 8501 to provide a measure of their resistivity; in a similar way, mechanically cleaned metal surfaces are represented by the nomenclatures, St 2 and St 3. Stripe Coating implies application normally by brush, of one or more coating layer on edges, welds or inaccessible locations to build up adequate total DFT at the actual locations. Occupational Health and Safety Administration (OSHA) related precautions There are a host of elaborately detailed OSHA Regulations which need to be complied with, in respect of any marine repair job undertaken, which encompass amongst several others, the following principal areas of concern based on preliminary risk assessment: Confined spaces; Personal Protective Equipment (PPE) at work; Construction – Head Protection; Fire safety; First Aid; electricity at work – electrical safety, quality and continuity; control of substances hazardous to health; all the Regulations cited in italics are the named few, relevant in this case of ballast tank repair. Lifting operations and lifting equipments; pressure equipment and pressure systems safety; work at height;; control of vibration at work; noise at work; dangerous substances and explosive atmospheres; display screen equipment; manual handling operations, Safety signs and Signals; Reporting of injuries, diseases and dangerous occurrences etc. are some other important Regulations which need to be also complied with, based on the assessed need; while some of the basic Regulations are mandatory, others are driven by the demands of the repair worksite. HSE (Health, Safety and Environment) Standards devised and enunciated based on years of experience and applicable to the marine industry could be termed the forerunner to the implementation of the OSHA Regulations. Selection of materials for repair of the ballast tank Apart from welding consumables chosen in tune with the material of construction of the ballast tank, coating materials chosen shall have bearing on the shape and configuration of the tank compartment and its function, including water quality which is likely to be contaminated or at times, even acidic. Light coloured coatings are recommended to facilitate inspections. Paint used with a less epoxy to tar ratio would warrant a higher thickness of coating. The measured dry film thickness (DFT) should not exceed the maximum DFT defined by the paint manufacturer. Coating products deemed adequate for the intended service and tolerant of the expected surface condition should be selected in co-operation with the coating manufacturer, with a proven track record of good product performance, and after-sale advisory and inspection services. Preparations for painting/coating of the repair For the purpose of illustration, the ballast tank with a target useful life of 10 years has been chosen, with two coats of a light epoxy based compound and a total measured nominal DFT of 300 microns complying with the 80/20 rule. The primary surface preparation shall comprise of zinc containing silicate based pre-fabrication primer on surface blast cleaned to minimum Sa 2.5 roughness. One stripe coat is mandatory before each full coat on edges, welds and in areas where spraying may not be fully effective. The secondary surface preparation shall envisage removal of sharp edges. While intact shop primer can remain, damaged shop primer should be blast cleaned to Sa 2.5 including welds and burns. Mechanical cleaning to St 3 is acceptable on block joints and damages to the applied coating system. Any visible salt contamination, oil, grease, dust, weld smoke or dirt on shop is to be primed or the other surface is coated; the chloride content on surfaces to be coated shall be within the limit set by the coating manufacturer. The thermal and hygrometric conditions during coating shall be with air humidity ? 85 % and steel temperature ? 3 o C above the dew point during blast cleaning and coating application operations. A coating specification drawn up in consultation with the manufacturer, wherein details regarding type, thickness, number of coats, in-situ coating facilities, availability of equipments for control of air humidity, temperatures and ventilation, steel surface treatment, coating application and curing, repair protocols and testing methods and acceptance criteria are firmed up in advance is absolutely essential. Planned maintenance in corrosion prevention for vessel structures As a thumb rule, maintenance coating should be carried out before rust engulfs 1 % of the surface area; ASTM D 610 is a useful standard for estimation of area percentages for arriving at a repair decision. Mud, sludge and foul water in the vicinity of the structures supporting the bottom of tanks should be removed on a planned and continuous basis to prevent pitting corrosion. Salts promoting early blistering of coatings due to osmosis should be removed regularly by means of fresh water washing. Adequate ventilation and dehumidification measures should be consistently implemented to avoid condensation of moisture on the steel surfaces. A minimum of 1 to 2 coats of 300-350 microns DFT, epoxy based, light coloured, surface-tolerant coating is recommended as a preventive maintenance coat for ballast tanks, hull internals and vessel structures as per international standards to even prevent galvanic corrosion. The efficacy of coating adhesion can be ascertained by inspecting it against the provisions made in ISO 2409. References 1. Akzo Nobel International Marine Coatings: Coatings Technology: What is Corrosion? Retrieved on 20 September 2011 2. Det Norske Veritas (DNV), Recommended Practice: Corrosion Protection of Ships, 2000, Retrieved on 20 September 2011 3. Chemco International: Ballast Water Tanks Repair Guide for Crew, January 2011, Retrieved on 20 September 2011 4. British Wind Energy Association (BWEA): The European Marine Energy Centre (EMEC); Guidelines for Health and Safety in the Marine Energy Industry, Retrieved on 20 September 2011 5. Det Norske Veritas (DNV), Classification Notes No. 33.1: Corrosion Prevention of Tanks and Holds, July 1999, Retrieved on 20 September 2011 Read More