^ Aluminum Hulled Boat (Source: http://www.aluminum-plates.com/a/Aluminum_plate_products/Marine_Aluminum_Plate/)
1Merits of Aluminum & Co.
Some two hundred years ago, the crew aboard British frigate Alarm noted that the iron nails holding the copper sheets on the underside of the vessel had rusted. This was less than two years after they had installed the protective sheets. This was the first recorded instance of galvanic corrosion.
Technology and materials used in shipbuilding has evolved with time, but the issue persists. Aluminum and its alloys are widely used to build hulls, hatch covers, and deckhouses of ships. The high strength-to-weight ratio of aluminum is its major attraction in shipbuilding, for it slashes final weight by 55-67% as compared to stainless steel (SS).
That apart, the excellent corrosion resistance, weld-ability, thermal and electrical conductivity, machine-ability, and recycle-ability of aluminum and alloys transforms into fuel savings, lower total cost of ownership and maintenance, greater speed, higher stability and maneuver-ability, and better range.
Marine applications commonly employ 5XXX series aluminum alloys viz. 5083, 5086, 5454, and 5456. Magnesium (Mg) is the chief alloying element for this series. Al-Mg-Si alloys and Al-Mg alloys with over 2.5% Mg resist corrosion in seawater. Studying aluminum corrosion is crucial in light of aluminum’s importance in shipbuilding.
What Makes Aluminum Corrosion Resistant?
Aluminum spontaneously forms a thin (50-100angstorm), inert, and protective oxide layer within seconds. The layer checks further oxidation and lends aluminum exceptional corrosion resistance despite its potent affinity for oxygen.
Moreover, the layer is impermeable, sticks strongly to the parent material, rapidly recovers from mechanical damages, and is stable in the pH range of 4-9. Beyond this range, the layer dissolves making aluminum vulnerable to strong alkaline solutions and inorganic acids.
Aluminum’s corrosion resistance drops in chloride environments. With greater percentage of salt (NaCl) than fresh water, seawater is more corrosive. The presence of copper and steel boost the chances of corrosion.
Types, Causes, & Remedies
Time of Wetness (TOW) and Composition of Surface Electrolytes are the two most important factors that determine corrosion of metals in open air. TOW is the duration for which the air to which the metal is exposed possesses a relative humidity of over 80% and a temperature of above 00C. TOW jumps to great levels inside water.
Aluminum is most likely to corrode on account of:
- Galvanic Corrosion
- Crevice Corrosion
Other mechanisms that corrode aluminum:
- Uniform Corrosion
- Exfoliation Corrosion
- Inter-granular Corrosion
- Stress Corrosion Cracking
- Corrosion Fatigue
Pitting is the most common corrosion affecting aluminum and its effects are more pronounced on untreated aluminum. It is so called because it forms minute pits on metal surfaces in the presence of electrolytes i.e. moisture or water.
Normally, pits start at flaws in the surface oxide films that cannot self repair. Pitting affects metals such aluminum, magnesium, steel, titanium, and copper and their alloys that form partially protective surface oxide layers.
Pits can be conical, saucer-shaped, or hemispherical. They never make deep inroads into the metal making them more of a visual issue than a structural one. Designing easy-drying metal surfaces nips pitting in its bud. Sacrificial anodes are an effective deterrent and so is surface treatment such as painting.
Galvanic Corrosion is the erosion of a metal when an electrolyte connects it to a less- electrochemically-active metal. Ships have a number of such metal combinations. Salt water normally plays the role of an electrolyte and completes the galvanic cell.
In such a setting, the more electrochemically active (less noble) metal becomes the anode (positively charged electrode) and its ions move towards the less active (more noble) metal i.e. cathode through the electrolyte causing metal loss at the anode.
Dissimilar metal combinations aboard ships include SS fasteners on aluminum fittings, bronze propellers on SS shafts, and attachments of diverse metals on carbon fiber spars. Welding or brazing different materials also creates such dissimilar junctions where corrosion currents are high. Such high currents make joints prone to galvanic corrosion.
Electrolytic Corrosion is slightly different from galvanic corrosion in that it requires external electric current in addition to two dissimilar metals connected through an electrolyte.
You can prevent galvanic corrosion through:
- Cathodic Protection uses sacrificial anodes i.e. less noble metals placed in direct contact with the more noble metal that needs protection. Magnesium and zinc play sacrificial anodes for aluminum
- Insulation such as neoprene prevents contact between diverse metals
- Interrupting the Galvanic Cell by painting the cathode. This is employed in large settings where electric insulation is impractical
- Combining Aluminum with Galvanized SS when an Al-SS combination is inevitable. Zinc is less noble than aluminum. For best results, use hot dipped galvanized SS only
Crevice Corrosion occurs in the tiny gaps between metal surfaces when water gets in here. Causes include:
- Differential Aeration
- Condensation due to temperature changes
- Capillary Action sucks in water
- Flawed Geometry of Parts retains water instead of draining it
- Improper Joints between Metal Surfaces
Aluminum in marine settings is prone to such corrosion because seawater provides moisture plus chloride salt. However, aluminum oxide formed from the corrosion limits the entry of water into the recess and halts crevice corrosion. Two-sided tapes and sealing compounds prevent such corrosion by restricting the entry of water.
Uniform Corrosion removes metal from all areas of aluminum at a similar rate. The metal finally fails due to thinning. Inhibitors such as chromic acid, appropriate coatings, and cathodic protection limit such corrosion.
Inter-granular Corrosion is galvanic corrosion at the microscopic level. It occurs at the grain boundaries between different phases inside metals and their alloys. Aged Al-Mg-Cu alloys are susceptible to such corrosion. Heat treatments that stimulate uniform precipitation lower inter-granular corrosion.
Exfoliation Corrosion crops up along numerous narrow grain boundaries and pries apart metal layers. Materials such as Al-Zn-Mg-Cu and Al-Mg-Cu alloys with acute grain structure directionality are particularly prone. It does not materialize in materials with equiaxed grain structures.
Stress Corrosion Cracking (SCC) is the trans-granular or inter-granular cracking of metals brought about by a combination of external conditions and static tensile strength. The greatest problem with SCC is metals crack suddenly without betraying any symptoms of a possible failure.
A combination of umpteen causes can trigger SCC. It is more common in high-strength aluminum alloys such as Al-Zn-Mg alloys placed under prolonged tensile stress in corrosive media. Most Al-Mg-Si alloys are immune from this disorder.
Corrosion Fatigue is the reduction in fatigue strength of metals due to corrosion and cyclic loading. All aluminum alloys lose between 65% and 75% of their fatigue strength (vis-à-vis fatigue strength in air) in sodium chloride solution at 108 cycles.
Advances in material sciences have always furthered the frontier of human civilization. Ships have come a long way from their wooden hull days. Steel is still the undisputed hull material but aluminum has its own niche space. With further developments, aluminum and its alloys will offer more desirable features.