Corrosion is as old as the earth, but it has been known by different names. Corrosion is known commonly as rust, an undesirable phenomena which destroys the luster and beauty of objects and shortens their life. A Roman philosopher, Pliny (AD 23–79) wrote about the destruction of iron in his essay ‘Ferrum Corrumpitar’.
Corrosion since ancient times has affected not only the quality of daily lives of people, but also their technical progress.
The term “Atmosphere Corrosion” comprises the attack on metal exposed to the air as opposed to metal immersed in a liquid. Atmospheric corrosion is the most prevalent type of corrosion for common metal.
Indian contribution to the science of corrosion
During the Gupta Dynasty (320–480 CE), the production of iron in India achieved a high degree of sophistication, as attested by the Dhar Pillar, a 7-tonne (7000 kg), one-piece iron column made in the fourth century CE. The existence of this pillar implies that the production of iron from oxide ore was a well-established process, and the personnel involved in the production of the iron pillar were aware of the reverse reaction involving the oxidation of iron to produce iron oxide (the familiar rusting of iron) and of the need to minimize the extent of this reverse reaction. Copper nails coated with lead were used by the Greeks in the construction of lead covered decks for ships. The Greeks Protection of iron by bitumen and tar was known and practiced by the Romans.
Iron pillar and sulphur
The sulphur content of the Delhi Pillar is very low, according to all determinations, probably because charcoal was used in reducing the ore. Coupled with the fact that the manganese content of the Pillar is also very low. The very low percentage of sulphur means that there are very few centres of iron-rich manganese sulphide (MnS) to initiate pit corrosion by serving as effective local cathodes. In fact, the sulphur printing technique has rarely revealed in the pillar iron any microscopically visible inclusions of sulphide. Thus the low sulphur and manganese contents are expected to make some contribution to the increase in corrosion resistance of the Delhi Pillar.
In thermodynamics, “Corrosion is the reversion or partial reversion from the meta stable condition of the metal to stable condition of its compound accompanied by the reduction in the free energy of the system”.
Sulphur dioxide (SO2):
The most important corrosive constitute of industrial atmosphere is SO2, which originates predominantly from the burning of coal, oil, gasoline and from the fossil-fuel power plants. Emissions of SO2 in Europe sharply increased after 1950 due to the rise of oil consumption and amounted about 25 million tons of sulphur by 1970. In 1968, an estimated 33~106 tons of sulphur oxides were emitted in the U.S.A.
The solubility of SO2 in water is very high (about 40 volumes of SO2 in 1 volume of water in ordinary condition). SO2 is non-flammable colorless gas having pungent irritating odour. The concentration required for taste detection ranges from 0.3 ppm to 1 ppm in air and the odour threshold is about 0.5 ppm. Both the chemical composition and the physical state of the pollutants change during their transport in the atmosphere. The life time of SO2 in the atmosphere is usually 1.5 to 2 days, which corresponds to a mean transport distance of a few hundred kilometers.
Rolls Royce issues with sulphur
Talking to journalists at a recent media presentation event, Rolls-Royce civil aerospace chief customer officer Dominic Horwood shed light on the causes of turbine blade cracking in the Trent 1000 gas turbine engine, which powers the Boeing 787 Dreamliner.
Cracking problems in the intermediate pressure (IPT) section of the turbine have plagued the engine since early 2016, five years after its launch. Unscheduled groundings of Trent 1000-powered aircraft cost Rolls-Royce some £450m last year, and Horwood said that addressing the problem was “the single most important issue” currently facing the company.
According to Horwood, the problem was caused by sulphurisation; a chemical process affecting the nickel alloy which comprises the IPT blades. “We are very confident that this problem will not occur in any of our other engines,” Horwood said. “This is confined to component level in the Trent 1000.”
Studies have shown that trace amount of H2SO4 vapour of industrial area seriously shorten the life of metal structures. The effect is most pronounced for zinc, cadmium and nickel. It is less pronounced for metals that are more resistant to dilute sulphuric acid, such as lead, aluminum and stainless steel.
Time of wetness is a key parameter, directly determining the duration of the electrochemical corrosion process. This variable is a complex one, since all the means of formation and evaporation of an electrolyte solution on a metal surface must be considered.
DURING COMBUSTION in the gas turbine, sulfur from the fuel reacts with sodium chloride from ingested air at elevated temperatures to form sodium sulfate. The sodium sulfate then deposits on the hot-section components, such as nozzle guide vanes and rotor blades, resulting in accelerated oxidation (or sulfidation) attack. This is commonly referred to as “hot corrosion.”
Sulfur in the fuel is generally limited to 0.3% for commercial jet engines. Sodium chloride comes from seawater. Seawater is also a source of sulfur. For aircraft engines, Tschinkel suggested that runway dust may be a source of salts.
Gas turbines generally use large amounts of excess air for combustion (a large fraction of air, in fact, is also used to cool the combustor).
These air-to-fuel ratios correspond to about 0.12 to 0.18 mole fractions of oxygen in the combustion zone. Thus, the combustion gas atmosphere
is highly oxidizing.
High-temperature alloys that suffered hot corrosion attack were generally found to exhibit both oxidation and sulfidation. The hot corrosion
morphology is typically characterized by a thick, porous layer of oxides with the underlying alloy matrix depleted in chromium, followed by internal chromium-rich sulfides.
Data shows that the corrosion of zinc and steel varies considerably from one atmosphere to another. The corrosion rate of zinc is lowest in dry, clean atmosphere and highest in wet, industrial atmosphere. Sea coast atmospheres, not in direct contact with salt spray are mildly corrosive to zinc.
On a positive note, more and more engine manufacturers should setup shop in India. The biggest aviation growth story is happening here and so is India the most challenging in terms of adverse atmospheric conditions.
Developing and testing engines in India will have a high degree of success as compared to the sanitised conditions of Europe or USA. Even though India has not been able to take advantage of its rich heritage in any field, its never too late to offer its adversities and let the world learn form it.