Potentiodynamic Scanning (PDS) of Stainless Steel 8-2-26 Supervision: Elien Wallaert
Introduction In order to investigate the corrosion resistance of Stainless steel in a specific environment, the practical class introduced the potentiodynamic scan method, in which a potential sweep is applied on a metal immerse in an electrolyte that simulates the environment of interest, registering the current density. To measure a certain potential of a metal you need a secondary system, i.e. one cannot just measure one potential, but a potential difference. A counter electrode is necessary for the measurement; however this cannot be made of metal due to its instability. The reference Standard Hydrogen Electrode (SHE) is the standard reference system, or universal zero potential, although it is very unpractical : the need of hydrogen gas, which is quite explosive hinders the usage. This is the reason why other stable systems are used instead, such as Silver/Silver Chloride. This system has its own stable potential which is known, and this information is used to measure the potential difference. The potassium chloride adds chlorides in the environment by being dissolved into water, generating salt caused by saturation creating a silver/silver chloride environment. Other systems like Mercury/ Mercury Sulphide are also extensively used. 2 Methods The evaluation of corrosion on stainless steel and alloys of transition metals is a widely used method, analyzing polarization curves in corrosive situations. To carry the experiment it is necessary to have a good connection, thus before measuring the metal, the top surface possibly composed of an oxide layer has to be removed, preventing the experiment to measure, for example aluminium oxide instead of the aluminium itself. The stainless steel plates shall be grinded with SiC paper under running water in one direction, and then tilt 9. Two types of austenitic stainless steel are used as electrodes: o AISI 34 (Cr, Ni) o AISI 36 (Cr, Ni, Mo) They are tested in an oxidizing environment contaminated with chloride ions. Two different solutions are used: o Aqueous solution H 2 SO 4 (.5M) o Mixture of H 2 SO 4 (.5M) and HCl (M)
3 Results 3. The shift of potential When the graph displays a lower potential than the potential of the material itself, it means that the material is being reduced. The reduction reaction will be the oxide layer being placed again in the solution letting the material free to the environment. On the open circuit potential, the potential goes higher until the potential of the material itself, when oxidation and reduction reactions go at an equal pace, thus no net current. An over potential is then applied, increasing the current and the oxidation reaction until certain point in which current stops to increase and goes into a steady state. On the passivation region, the graph will display the passi vation current, which means that the material is passivated. It has no problem with the potential applied, being just stable. 3.. Which reactions can occur during the polarization? Consider the working and counter electrode separately. Why do we use a Pt-grit as counter electrode? Anode: on the working electrode, where oxidation is carried, the following reactions happen: Fe Fe 2+ + 2e Cr Cr 3+ + 3 e Ni Ni 2+ + 2 e Mo Mo +4 + 4 e The optimization of the experiment would be to avoid the first reaction to ha ppen, since it is not interesting that iron oxidizes. The reaction with molybdenum can naturally only occur in AISI 36. 2 The reactions involving nickel, chromium and molybdenum lead to the formation of an oxide layer labeled passivation layer. Cathode: On the counter electrode the following reduction reactions were carried: 2H + + 2e H 2 4H + + O 2 + 4e 2 H 2 O The second reaction only happens for AISI 36 and with the condition of a high potential. The counter electrode used for voltammetric analysis is of Platinum, an inert noble metal that does not react with other components in the solution, thus being highly corrosion resistant. On the experiment, the Platinum electrode has the shape of a grid, which means the specific area is enlarged and the electrolysis will not be limited by reduction reaction.
Potential V 3..2 Consider the PDS-curve measured for the AISI 34 in.5m H 2SO 4. Why does the curve deviate from the TAFEL-slope? A passivation layer is formed on the surface, i nhibiting further corrosion reaction. Because of the phenomenon of passivity, the current can change up to six orders of magnitude during the experiment. This current is the sweep measurement done by the potentiostat. At around -.2 volts a sharp edge can be seen in the curve, indicating where the current reverses polarity as the reaction becom es cathodic instead of anodic, or vice versa, and happens because of the logarithmic scale (Figure ). H 2 SO 4.4.2.8.6.4.2 -.2 -.4 -.6...... Log(current density) 34 3 Figure : Corrosion process. The vertical axis is the electrical potential, and the horizontal asis is the logarithmic of absolute current. The use of logaritmic axis is due to the wide range of current values that must be recorded during the experiment.
Potential V Potential V 3..3 Mark the following properties/phenomena on the curve: o The critical current density for passivation i c o The passive region o The passive current density i p o The region of active dissolution o Oxygen gas formation o The cathodic Tafel line o The anodic Tafel line.2.8 H 2 SO 4 +HCl Oxygen Gas Formation.6.4.2 -.2 -.4 Passive Region Region of Active Dissolution Anodic Tafel Line -.6 Cathodic Tafel Line -.8...... Log (current density) 36 4.2.8.6.4.2 -.2 -.4 -.6 Passive Current Density I p H 2 SO 4 +HCl I c Critical current density -.8...... Log (current density ma) 36
Potential V Potential V Potential V Potential V 3..4 Superimpose the PDS curves on one another and compare them. How would you explain the differences? The values can range below zero (.65) towards an increase which indicates passivation, ending in.2 volts. The value below zero is due to the Silver/ Silver Chloride. On figure 2, the line corresponding to AISI 36 displays a nose like behavior of the graph signaled in yellow. However, it is rather small, which indicates that even though in 36 some energy had to be used to passivate the sample, it was not very high. This small difference is caused by the molybdenum in the composition of t he stainless steel. On figure 3 the noses can be seen, indicating that a lot of energy was necessary to passivate the layers. We can then conclude that when the stainless steel is immersed in a solution with chloride ions, it experiences more difficulty to form the layer which will prevent against corrosion. On figure 4 the most notable features are the shortness of the passive region. The critical current density for passivation is much larger for solutions with HCl, thus a higher corrosion rate. The chloride ions cause a phenomenon called pitting, which means that more energy will be needed to passivate the layer. H 2 SO 4.4.2.8.6.4.2 -.2 -.4 -.6 E-8... 34 36 Log(current density) Figure 2: On this graph, one can see that the AISI 36 has a typical nose circled in yellow that indicates that more energy was necessary to passivate the sample. H 2 SO 4 +HCl.2.8.6.4.2 -.2 -.4 -.6 -.8 -... 36 34 Log (current density) Figure 3: The values of the current are clearly different. AISI 36 has molybdenium in its composition, which prevents corrosion, and lower corrosion is related to lower current density. 5 AISI 34.5.5 -.5 -... Log(current density) H2SO4 H2SO4+HCl AISI 36.4.2.8.6.4.2 -.2 -.4 -.6E-8... -.8 H2SO4 H2SO4+HCl Log(current density) Figure 4: When in a solution with HCl, AISI 34 and AISI 36 corrode a lot more than in only H 2 SO 4 solution. For this reason, the curve always present higher values for current for the system with chloride. The lack of the nose is evident for both cases without chloride.
3..5 Calculate the polarization resistance R p and the Tafel parameters (from the cathodic and the anodic parts) from both steel grades in every electrolyte solution. Compare the results and discuss. Which conclusions can you make concerning the corrosion resistance of the stainless steel grades 34 and 36 in contact with chloride-ions? During the practical class, the computer program for the potentiodynamic scanning was used in order to measure I corr, by drawing Tafel lines on the graphs and extracting the data regarding the intersection of anodic and cathodic Tafel line. Changing the axis from logarithmic to linear, one should be able to extract the values for the slope of the curves R p. I corr (μa) R p (kω) AISI 34 H 2 SO 4 25,58,52 AISI 36 H 2 SO 4 44,686,493 AISI 34 4%H 2 SO 4 /6%HCL 79,574 97,442 AISI 36 4%H 2 SO 4 /6%HCL 8,669 25,65 Table : Corrosion current and polarization resistance for stainless steel in different environments. The value assigned in red had a clear missmatch with what was expected. During the measurements, AISI 36 which contains molybdenum is expected to have higher resistance to polarize and lower current. However, due to probable measurement problems either on the sample preparation or extraction of data, the value obtained shows discrepancy with literature (Table ). The reason for the higher resistance is due to the fact that the molybdenum in the composition increases the efficiency of the p assivation layer made of oxide. The reason for the lower current shall be the effect of the passivation layer as well, preventing the corrosion from happening. 6 The corrosion current is a lot higher when there is HCl in the solut ion, which indicates the relationship with Chloride ions and pitting. Regarding the polarization resistance, it should be inversely proportional to the corrosion resistance. The more resistant to corrosion, the higher the polarization resistance and the lower the corrosion current. Thus, R p of AISI 34 in the solution with HCl should have been the lowest value, followed by AISI 36 also in HCl solution, since these materials are more likely to corrode.
4 Conclusion Alloys can be ranked according to their corrosion re sistance. To determine the differences regarding alloys and impurity elements, different corrosion parameters are analysed. When 34 or 36 stainless steel are passivated in sulphuric acid media containing chloride ions, pitting corrosion can be observed. Molybdenum indeed prevents corrosion as it improves the oxide layer responsible for passivation of the material. Sometimes the measurements can be unstable regarding stainless steel, because of layers of chromium and nickel oxide. This creates the passivation layer which is very important for stainless steel, although being a fragile part: when in contact with chloride ion, the oxide layer loses its stability. This is the reason why two environments were selected, one with aqueous solution of sulfuric acid, and another being a mixture of sulfuric acid and hydrogen chloride in which the chloride ion is added. Afterwa rds, the difference between environments was analysed, and also the difference between materials, however the measurements were inconclusive. The most likely reason for inconclusive results is the inefficiency in removing t he remains from previous experiments from the stainless steel samples, by not grinding it properly. 7