The factors affecting are categorized into two
- Primary Factor (those dependent on metal)
- Secondary Factor(those dependent on environment)
The primary factors are something that we can’t change or control easily because they are due to chemical properties of metal itself e.g.
- Nature of metal – The galvanic series position i.e. the tendency to get oxidize say a metal have higher tendency to oxidize the easily it will for ions and so that easily will the rust or corrosion process will occur.
More is the oxidizing tendency, easier is the corrosion
- Surface and shape – The surface state of the metal is a important factor, say what we do to control is putting a layer of oil or grease to prevent the contact of moisture as different shapes or metal have a different speed of corrosion because of the shape only, at pointed edges we have the required anodic condition of differential aeration type of corrosion(later) etc
Heterogeneity and sharp edges facilitate the corrosion process
- Protective film layers – Many metal have a tendency to react to the corrosive environment and form a protective layer of their oxide majorly, now this is something that act as a passive coating to the metal, not all but the metal possessing this property namely Al, Ti and Cr form.
A passive layer of oxide reduce corrosion process drastically
Similarly there are other properties like over-voltage and potential difference which we will cover in upcoming topics such as metal finishing and control methods.
The next type is something we can actually control the factors based on the environment of the metal while corrosion, all factors have a simple contribution to the any two of the reaction we discussed and we will use simple equilibrium principle to get through these properties and understand in what way they are affecting the rate of corrosion
- pH – the simple explanation can be given by equilibrium principle of the following reaction
2H+ + 2e ——> H2 (g)
Here if we increase pH i.e. decrease the concentration of H+ the rate of consumption of electron is decreased so rate of oxidation or to say decomposition of metal is decreased while is we get into acidic medium the exact reverse happens therefore
Corrosion rate decreases with the increase of pH
- Temperature – The reaction here all are increase by the rise of temperature and therefore we have a higher chances of corrosion in a heated environment considering all other factors to be same
Corrosion rate increases with increase in temperature
- Anodic to cathodic area ratio- the ratio of area matters a lot and is one of most common methods of corrosion control, the simple principal we here use is that the cathode reaction can only occur when cathode have a much larger area that anode because it needs a contact to oxygen and moisture an d more is surface higher is rate and amount of contact
Here we have Fe which is anode here, now say we have here a small narrow hole then the rate of corrosion will be very drastic because the cathode here have enough surface to proceed its reaction and for the anodic half of reaction we don’t need anything but a contact to the other half because e can move from inside as well, but in case below
The area provided to the cathode to react with moisture ,But the lower half Sn is cathode, now if we get a crack here the upper half which is Fe is anode again
Oxygen is very less and there for it will be there but at a
Much slower rate therefore we are going to use such techniques in galvanizing where we actually much such things cover the metal which acts as anode and since they have more area they don’t let the metal below it getting corroded.
The greater the cathode to anode area is faster is the corrosion
- Humidity –Water or moist being a necessary component of corrosion plays the important part and is directly proportional to the rate of corrosion until a certain humidity called critical humidity, while after that id abruptly increases with increase in relative humidity
The rate of corrosion increase with increase with relative humidity of surrounding
- Conductance – the conductance of the medium have a role to play in the processes, i.e. movement of electron, although most of metal do it in no time but if talk relative to all kind of material the one with higher conductance definitely have a edge here, additionally it’s not only the metal but other ions moving, moisture and all of them making a conducting environment helps the electrochemical process of corrosion.
The rate of corrosion increase with higher conductance of the medium
- Nature of Corrosion Product – Since the factor of area ratio is already discussed the higher the oxygen and moisture contact higher is the rate of corrosion
additionally when we talk about the product acting like a cover of rust over the surface we have it of two type porous or non- porous and since we need contact with the two essentials we need it to be porous to promote and non-porous to restrict the corrosion process therefore,
Corrosion is at higher rate if product is porous; if not porous the corrosion is slowed down
There are some factors like polarization which will be discussed in metal finishing (later)
Abhishek kumar jha
(Chemistry at Utkarshini)
3-103-factors-influencing-corrosion note.doc file to save.