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Corrosion Resistance

Metals in service often provide a superficial idea of permanence and all metals except Platinum and Gold are chemically unstable when exposed to air and moisture laden air. At ambient temperature, most of these metals become highly unstable even without moisture. In some metal or environmental system the metal is protected by passivity. These can be either naturally formed surface condition or other forms which inhibit reaction.

In other systems the metal surface remains active and some form of protection is provided by either design or method of reagent treatment to help inhibit the reaction with atmospheric oxygen to prevent corrosion. Corrosion begins to occur when protective mechanism are overlooked, break down or are exhausted leaving the metal vulnerable to attack.

Degradation by corrosion, or wear and tear can only be solved by considering a metal not in isolation but within a wider system with the components, metal, chemical stress and time. Hence, a metal selected to serve well in one particular chemical environment or stress system might not work for the other and will be inadequate. 

Some of the features of performance expected from metals and metal artefacts can also be predicted by their intrinsic characteristics derived from their compositions, structures as well as thermal and mechanical features. Corrosion resistance helps in identifying these characteristics in a systematic manner.

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Corrosion Resistance Definition

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It is well known that unprotected iron begins to rust in pure neutral water but only in case this has dissolved oxygen. So the standard method of controlling corrosion of these materials is by removing the oxygen from water. 

Corrosion basically is chemical degradation of metals due to adverse environmental conditions of excess moisture forming the oxides of the metal on surface and slowly wear away the metal surface in layers. Methods, metal combinations and treatments that are employed to help resist this process of corrosion is typically considered as corrosion resistance. 

These are not very consistent with the current idea of corrosion as copper has good resistance to neutral water irrespective of the presence of oxygen. Combining metals in specific ratios to form alloys also help in resisting corrosion. Most of the applications, even though conflicting are the underlying corrosion processes and protection strategies.

The corrosion process is basically controlled by two spontaneous coupled chemical changes which take place at the interface of metal and an aqueous environment. 

The first reaction is when the chemical agents from the given aqueous surroundings help remove electrons from the metal and the second one is the reaction where the atoms of metal surface take part in order to push back electrons lost during the first reaction. 
In other words, it’s a combination of both oxidation and reduction reactions and redox change.

Corrosion usually interact with a surface of a stressed metal to create fracture at the critical point of only fractions of the normal fracture stress. 
These effects can be life threatening if these occurs in a flying object. 

Corrosion fatigue failure can affect any metal and is a type of fracture at a low stress as the result of cracking propagated by cycling loading. 
The failure is delayed and the effect is accommodated in design by assigning for a given applied cyclic stress corrosion cracking.

Certain strong passive metals such as stainless steels and some non-metallic conductors like graphite can simulate noble metals. The effect is to intensify attack on the less noble of the pair of metals in electrical contact exposed to same aqueous environment.
  • The unprotected regions of most of the active metals which are exposed to environment dynamics, are definitely vulnerable but the overall corrosion can be delayed or prevented by various natural or artificial applications
  • Steels corrode actively in moist air and water containing dissolved oxygen pr even by deposition by chalky or other deposits on metal surface from natural waters.
  • Cathodic protection provides a method of protecting active metals in continuous contact with water as in case of ships and pipelines. This also definitely depends on metal dissolution reaction with an specific electrical potential applied with the help of impressing a cathodic current.
  • In case metal protection methods are either inappropriate or uneconomic, then the active metals neeeds to be protected by applying by organic coatings such as alkyd and eposxy resins.
  • The range from oil based, air drying paints applied by brush or to thermosetting media dispersed in water for application by electro deposition to manufactured products like vehicular bodies.

Corrosion Resistance Materials

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Erosion or corrosion resistant materials find use in many applications where the surfaces are subjected to eroding forces at higher temperature. Reactor vessels and piping used for high temperature processes are exposed to aggressive liquids or reagents which often contain catalytic materials and protection against these agents needs technological edge. 

In recent times the monolithic refractory liners are used for components which require adequate protection against severe erosion and corrosion such as inside walls of steel plant furnaces and acid vats used in tanneries.

Corrosions in turbine engines, boilers and furnaces for waste disposal occur on a regular periods and contrary to high temperature oxidation hot corrosion occurs with presence of a molten salt film on alloy surfaces.

Since molten salt is an electrolyte, the corrosion pattern is more likely to be electrochemical process. Most of the hot corrosion that happens in industrial environment is not the bulk molten salt but the existence of a thin molten salt film that causes the accelerated corrosion at high temperatures.

Diffusion of atmospheric gases is understood to be much faster under molten salt film conditions, the corrosion resistance or corrosion mechanism varies from bulk molten salt environment. Alloy corrosion rates changed greatly with the change of atmosphere or the change of oxygen partial pressure.

Nickel passivity
  • The passive film which forms on nickel will not protect the nickel from corrosion attack like nitric acids etc from atmosphere.
  • When nickel is alloyed with chromium, a stable form of film is produced and this makes a better corrosion resistive variety of oxidising media. However, these alloys are prone to environment attack which contains chloride and other halides.
  • Corrosion in such cases is seen in form of pitting. The addition of molybdenum or even tungsten improves the corrosion resistance.
  • Titanium passivity
  • Titanium forms a stable protective strong adhesive oxide film. This film forms quite instantly, when a fresh surface is exposed to air or moisture.
  • By addition of alloying elements to titanium affects the corrosion resistance because these elements changes the composition of the oxide film.
  • The oxide film of titanium is thin and is attacked by only a few substances and most notable of these are hydrofluoric acid and because of its strong affinity for oxygen, titanium is capable of healing ruptures instantly, if there’s a trace of moisture or oxygen is present.
When two different metallic salt materials are connected electrically through either electrolyte or by any other means which helps in transferring electron charges across then  an electric potential is developed and this potential difference provides a strong driving force for dissolution of the less noble material. It tends to reduce more noble material to dissolve as well. 

Crevice corrosion
The crevice corrosion is kind of localised corrosion occurring within or adjacent to narrow gaps or openings formed by metal to metal or metal to non-metal contacts. These results from the localised differences existing in O2 concentrations, associated deposits on the metal surfaces, gaskets, lap joints or crevices under a bolt or around some rivet where moisture can collect. The critical crevice corrosion temperature of an alloy is that temperature which crevice corrosion is first observed when immersed in a ferric electrolyte. 

Pitting 
Pitting is a form of localised corrosion which is primarily responsible for iron and steel hydraulic structural failures. Pitting may result in the perforation of water pipe which makes it unusable even though relatively small percentage of the total metal has been lost due to rusting. The initial process of pits are basically the breakdown of protection on surface. 

The main reasons for pitting acceleration is electrical contact between dissimilar metals or between concentration cells as these couples cause difference in potentials and finally result in abrasions in metal surfaces.

Corrosion Resistance Test

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As early as 1943, salt spray was used to check the corrosion resistance of zinc coatings. It was confirmed that salt spray does not reveal initial porosity in the zinc layered metal and then it provided only a mean of estimating the coating thickness. This test was meant for sacrificial metals. 

Carter in 1972 confirmed that a plentiful supply of well conducting electrolyte could lead to erroneous results by comparison with atmospheric exposure and alternate wetting and drying of the surface in a poorly conducting electrolyte.

Carter also affirms that comparative performance of zinc and cadmium varies with composition of salt solution used for spraying. 
Superiority of cadmium is lost when natural sea water, which contains magnesium salt, is substituted for a pure sodium chloride solution. 
  • Salt solution of 5% NaCl at ambient temperature for quarter of an hour.
  • Further exposure at room temperature for about 75 minutes
  • Humidity cabinet for 22.5 hours
  • Exposure to relative humidity of about 85%
The outdoor tests resulted in distance blistering of painted coatings and is considered as inversely proportional to zinc coatings thickness. Evaluation by Rendahl revealed that zinc and zinc alloy coatings have lower exposure to marine moisture. This concludes the idea of the sacrificial nature of zinc metals as coatings.

Corrosion Resistance Metals

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Aluminium is a typical example of a metal endowed with the ability to establish a naturally passive surface in appropriate environments. Aluminium tends to react with atmospheric oxygen and form a protective coatings of aluminium oxide $(Al_{2}O_{3})$

The protection given by this condition is very effective that aluminium is now considered as the primary metal material for cooking purpose. In some system, easy passivity characteristics can also be attributed for a dominant component. The same approach is used for formulating stainless steel percentage composition where, chromium is the main component inducing passivity.

Corrosion Resistance of Steel

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Iron and steel are the most versatile, least expensive and most widely applied. They have unequalled range of mechanical and physical properties with which they can be given alloying and heat treatment.

The main disadvantage of iron and its alloys is that they have poor resistance to corrosion even under mild environmental conditions. Pure iron exist as BCC or body centered cubic  structure at temperatures below 910 C and above 1400 C nbut in intermediate temperature range, it exist as FCC or face centered cubic structure.

Solutions in BCC are called ferrite while solutions in FCC are called austenite. The solutes stabilize the phases in which they are most soluble. 
Based on the pattern of composition we get to see various structures of steel.
  • Eutectoid composition: transforms from austenite to colonies of fine alternate ferrite and cementite
  • Hypo eutectoid composition: steel with lower carbon content
  • Hyper eutectoid composition: steels with higher carbon composition
Stainless steel are alloys of iron and chromium with or without other added components. These are most widely applied and versatile of the corrosion resistance alloys from anodic passivating metals. 

The alloys like stainless steel alter relative stabilities of austenite and ferrite. Carbon, nickel, manganese, nitrogen and copper stabilise austenite. Whereas, chromium, molybdenum and silicon stabilise ferrite.

They can help precipitate any carbon which are present in these compositions. Metals that form carbides selectively in steels include chromium, titanium and molybdenum. 
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