Oxidizing or reducing atmosphere?

[CdotS]

I have come across many papers which talk about Material A is good for oxidizing situations whereas Materila B is good for reducing atmospheres. What is the simplest method to identify/catagorize a corrosive environment - either oxidizing or reducing? Is it based on what happens to the metal surface when exposed to that specific environment? In oxidizing, metal gets oxidized and in reducing metal oxide get reduced back to metal? If there is no oxygen involved, how do you classify the mechanism?

Any good reference book to understand this concept?

 

[mcguire]

In non-aqueous environments it's pretty straightforward. Oxidation is the reaction in which the metal combines with some other element, oxygen, sulfur, chlorine, etc. and in doing so gives up and electron. Reduction is the same thing in the opposite direction.
The parallel exists in aqueous environments. A metal being corroded, or the site on the surface being attacked, is the anode. The metal is giving up at least an electron and likely going into solution.(if the oxidized species is soluble, corrosion can procede without hindrance of a possibly protective oxide.)
An environment is called oxidizing if the metal either becomes a compound, like Fe3O4, or becomes a cation, Fe+++.
If there is a species in the environment which can reduce the cation, such as chloride ions, which can cause the reaction Fe+++ plus electron = Fe++, the environment can be called reducing, although this is clearly not meant to mean the metal isn't still being corroded. It is a term used to imply that there are competing reactions occuring in a solution. If you add a reducing agent to an oxidizing environment such as Cl- to HNO3, the corrosion rates greatly increases because the reducing ion,the chloride,destabilizes the metal oxide.
Read Chapter 3 in Sedriks' "Corrosion of Stainless Steel.
HNO3 and H2SO4 are considered oxidizing; HCL is considered

 

[CdotS]

McGuire,
Many thanks for a neat explanation on the basics of oxidation and reduction, bringing my Uni days back to life!

One more question: Why concentrated H2SO4 is oxidizing whereas the dilute H2SO4 is reducing?

I do not have a copy of the above book but I should be able to check in the ASM Handbook on Corrosion, if it has any reference to oxidizing/reducing conditions.

 

[mcguire]

I'm assuming that this is in reference to carbon steel, in which case the dilute H2SO4 attacks the steel as one would intuitively expect, but high concentrations Fe ions don't go into solution. The anodic reaction ceases ( or doesn't start). The carbon steel replicates the behavior of stainless in that it behaves passively to this environment. Add some Fe or Cl ions, however, and watch corrosion take off.