how does a base speed up the rate of rusting???

Question

i kinda have an investigatory project at school...it's about the effect of pH level in the rate of rusting ....it seems that the base speeds up rusting....plzz help me..

Answer 1

The process of rusting (iron corrosion) is very complex. Most bases tend to accelerate the oxidation of the Iron atoms to Iron (II) ions. This is one of the first steps in the process.

Answer 2

Lancenigo di Villorba (TV), Italy

IT IS TRUE, YOUR QUESTION IS VERY HARD!

GENERALITIES
Since I think you refer to easiest experiments about "Iron & Mild Steel's Wet Corrosion", I refer to small iron pieces dipped in aqueous media. These quiet systems involve usually "Oxygen Uptake" 's phenomena, that are the soaking of air's Oxygen by the liquid in the test's tubes. As you will see, Oxygen transfer from the air toward the liquid media is a very important phenomenon.

WHAT IS RUST
Iron acts as reducing agent in several physico-chemical phenomena. The latter takes place since the hydroxide compounds of iron are VERY FEW SOLUBLE compounds in aqueous media. Chemically speaking, industrial and home waters favour rust forming since they contain several calcium salts, thus calcium concentrations increase the formation's rate of calcium/iron sedimented compounds or the RUST.

THERMODYNAMICS ON MEDIA's pH
M. Pourbaix was a belgian chemist (he born in Russia) who spent his academic life in researches about metal corrosion in wet surroundings, mainly iron and steels. It is well-known the "Pourbaix's Diagram for Iron", a valuable tool to understand fundament on chemical stability of ferrous matters.
WHAT DO YOU SEE IN IT?
The diagram put "Nernst's Electrical Potential" as Y Coordinate versus "Medium's pH" as X Coordinate, thus it tracks several lines closing some zones. Any zone is related to a chemical species, that is the "stability domain" of the chemical stuffs related, e.g pure iron or one of its compounds (e.g. among the latter you retrieve soluble salts as the Fe(II) or Fe(III) ions, you retrieve some oxides as "hematite, Fe2O3", "goethite, Fe2O3.H2O" or "magnetite, FeO-Fe2O3").
HOW DO YOU USE IT?
Briefly, you may remark three MAIN ZONES in this diagram, e.g. LOWEST, MEDIUM and HIGH pH's conditions.
In the LOWEST ZONE you can retrieve only Fe(II), Fe(OH)2 or Fe(III), that are distinguished on basis of the applied Electrical potential. Electrical potential is usually decided by the oxidizer acting in the medium. Since the common lab's experiences leads NOT to ferric ions, you noted the following facts. The experiment start showing that HYDROGEN's development runs vigorously ; in the succeeding times, the OXYGEN's uptake happens faster as GREATER is the Acidity's Grades. Thus, iron corrodes losing soluble and coloured ions : iron surface remain bright while medium assume greenish coloration.
When the system overcame to great ion's concentration, medium darkens itself toward Fe(III) ions while grey-brownish bodies precipitated, e.g. Fe(OH)3.

In the MEDIUM ZONE you can retrieve only magnetite,
e.g. FeO.Fe2O3. I remember that Electrical potential is usually decided by the oxidizer acting in the medium. Thus, iron corrode losing soluble and coloured ions : iron turns dark while medium may or not assume pale coloration. The kinetic governing the phenomena states a low corrosion's rate since OXYGEN's uptake is not favoured by "Low Acidity's Grade". The lower the acidity, the lower the global reaction's rate, the greater the probability of "Hydrated Oxide" 's formation, e.g. FeO.Fe2O3. The slow ion's release may determine their succeeding conversion above the metalic surface itself. In this manner OXYGEN's uptake slows down the global reaction's rate leading to dark "Hydrated Oxides" on the iron's surface. Besides Fe(II) ions there is also Fe(OH)2 or FeO.Fe2O3 : the system prefer the latter the lower are the acidity's conditions.

In the HIGH ZONE (e.g. ALKALINE MEDIA) you can retrieve mainly Fe(OH)2 or Fe(OH)4- salts, that are distinguished on the basis of Electrochemical Potential applied. I remember that Electrical potential is usually decided by the oxidizer acting in the medium. Thus, iron corrode forming unsoluble coloured sludges : iron turns greyish while medium remains uncoloured. The kinetic governing the phenomena states a low corrosion's rate since "Oxygen's uptake" is not favoured by "Alkalinity's Grade". The greater the alkalinity, the lower the global reaction's rate, the greater the probability of hydroxide's formation,
e.g. Fe(OH)2 when they form not ferrate salts. The slow ion's release may determine their succeeding conversion above the metalic surface itself. In this manner HYDROGEN's development "backs in action" (slowly) instead the oxygen uptake which happens not. The common lab's experiences cannot lead to ferrate ions, thus you obtain great GREY SLUDGES's MASS.

I hope this helps you.