Question

Give examples why some nail showed corrosion at the air/water interface. An in ground petrol storage tank at an ADNOC station is buried at depth so that sites partially below the water table. Over a period of time where would you expect most external corrosion of the tank? Give reason for your answer Explain the reason for the different amounts of corrosion in the nails in tap water, distilled water and boiled distilled water · » If corrosion of the nail in sodium chloride solution is an electrochemical (oxidation/reduction) process involving oxygen write half equations to show how iron corrodes. There is no corrosion of the nail in sodium hydroxide solution. Why? Corrosion of the nail in hydrochloric acid solution does not involve oxygen in the corrosion mechanism. USE half equation to explain why

Answer 1

• Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually metals) by chemical and/or electrochemical reaction with their environment .In other words Corrosion can be defined as the deterioration of materials by chemical processes. Of these, the most important by far is electrochemical corrosion of metals, in which the oxidation process M → M⁺ + e^– is facilitated by the presence of a suitable electron acceptor . In a sense, corrosion can be viewed as the spontaneous return of metals to their ores; the huge quantities of energy that were consumed in mining, refining, and manufacturing metals into useful objects is dissipated by a variety of different routes.
• A corrosion system can be regarded as a short-circuited electrochemical cell in which the anodic process is something like :

  Fe(s) → Fe²⁺(aq) + 2 e^–

  and the cathodic steps can be any of

  O₂ + 2 H₂O + 4e^– → 4 OH^–

  H⁺ + e^– → ½ H₂(g)

  M²⁺ + 2 e^– → M(s)   where M is a metal.

1. Nail show corrosion at air & water interface because it gets both moisture & oxygen which is basic recquirement for corrosion to occur.
2. In petrol tank corrosion is more prevalent in bottom corners of the tank due to dirt and water entrapment.
3. There is different amount of corrosion in tap water , distilled water & boiled distilled water because in distilled water there is lesser no of salt impurites so its corrosin rate will be lesser as compared to tap water . On the other end as temperature increases the rate of corrosion will increase ,
4. Sodium chloride increases the conductivity and generally increases the rate of reaction. The concentration of sodium chloride will determine the corrosion mechanism taking place.Increasing the concentration decreases irons pitting potential or in other words; pitting corrosion is more likely to occur in higher concentrations of sodium chloride opposed to uniform corrosion.
5. In the presence of hydroxide, there is no O₂ to react with so no reaction occurs, which explains the lack of corrosion . On the other hand If the pH is low (acidic), there will be a lot of H⁺ ions and few OH^- ions to neutralize or balance them . The H⁺ ions will tend to react with metals, giving up their positive charge, giving it to the metal so the metal can oxidize (have its oxidation state raised) and go into solution .
6. If the acid is hydrochloric acid, the reaction will look like this:

2H⁺Cl^- + Fe⁰ => H₂ + Fe⁺⁺Cl^-₂