Corrosion of Steel Lab Report
1. Results Part 1:
When samples of austenitic stainless steel are immersed in a sulphuric acid solution and are taken through a current, it may be observed that corrosion products are formed around the electrodes, to differing degrees. The reason for this is because when a uniform voltage is applied across the series of interconnected electrodes in the acid solution different potentials are obtained at each electrode and hence different electrochemical reactions take place. The products of these reactions can be identified by using complex ion indicators that assume a certain colour depending upon the type of cation present.
At electrode 1, it was observed that bubbles were being formed. The reaction occurring at this electrode is a cathodic reaction and as such the bubbles can be deduced to be H2 gas from the reactants involved.
2H+ + 2e- → H2(gas)
No discolouration of the indicator means that no corrosion was occurring at this electrode.
Electrodes 2 through til 5 showed no visible reaction. From inspection of a Pourbaix diagram it can be seen that for a pH of 2, Fe is in the cathodic immunity region where no oxidation occurs, which is why no reaction is observed.
Electrode 6 was seen to turn a distinct blue colour due to the presence of Fe2+ ions. This is because the potential involved in this oxidation reaction is in the active region.
Fe → Fe2+ + 2e-
The potential at electrode 6 is not high enough to form a protective Cr2O3 layer, as the Cr does not have sufficient energy to oxidise to Cr(III), therefore it corrodes.
There was no visible reaction observed between electrodes 7 through til 22. This is because the potentials involved in these reactions are in the passive region. The Cr in the electrodes creates a protective layer of Cr2O3 which slows down the corrosion of the electrodes to the extent that it is unnoticeable to the naked eye.
A pink discolouration was observed at electrodes 23 and 24, which indicates the presence of Fe3+ ions.
Fe →
When samples of austenitic stainless steel are immersed in a sulphuric acid solution and are taken through a current, it may be observed that corrosion products are formed around the electrodes, to differing degrees. The reason for this is because when a uniform voltage is applied across the series of interconnected electrodes in the acid solution different potentials are obtained at each electrode and hence different electrochemical reactions take place. The products of these reactions can be identified by using complex ion indicators that assume a certain colour depending upon the type of cation present.
At electrode 1, it was observed that bubbles were being formed. The reaction occurring at this electrode is a cathodic reaction and as such the bubbles can be deduced to be H2 gas from the reactants involved.
2H+ + 2e- → H2(gas)
No discolouration of the indicator means that no corrosion was occurring at this electrode.
Electrodes 2 through til 5 showed no visible reaction. From inspection of a Pourbaix diagram it can be seen that for a pH of 2, Fe is in the cathodic immunity region where no oxidation occurs, which is why no reaction is observed.
Electrode 6 was seen to turn a distinct blue colour due to the presence of Fe2+ ions. This is because the potential involved in this oxidation reaction is in the active region.
Fe → Fe2+ + 2e-
The potential at electrode 6 is not high enough to form a protective Cr2O3 layer, as the Cr does not have sufficient energy to oxidise to Cr(III), therefore it corrodes.
There was no visible reaction observed between electrodes 7 through til 22. This is because the potentials involved in these reactions are in the passive region. The Cr in the electrodes creates a protective layer of Cr2O3 which slows down the corrosion of the electrodes to the extent that it is unnoticeable to the naked eye.
A pink discolouration was observed at electrodes 23 and 24, which indicates the presence of Fe3+ ions.
Fe →
References: http://www.corrosionclinic.com/corrosion_online_lectures/ME303L11.HTM http://electrochem.cwru.edu/encycl/ http://www.corrosion-club.com/lpr.htm