Experiment-7Aim of the ExperimentPit Density : To find pit density along the given area of stainless steel sample. Theory: Pitting corrosion, or pitting, is a form of localized corrosion that leads to the creation of small holes in the metal. The driving power for pitting corrosion is the depassivation of a small area of the specimen, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions. The mechanism of pitting corrosion is probably the same as crevice corrosion. Mechanism Pitting can be separated into two different regions, namely pit initiation and pit growth. The growth mechanism is reasonably well understood, while initiation mechanism is not very clear. Pit Initiation Pit initiation is not well understood. Pit initiation time can vary from very short, days, to very long times, many years. Small changes in conditions can make the difference in whether pits occur or not. There are many mechanisms of pit initiation. The initiation mechanism could be metal specific and history dependent in some cases. In other situations a general type of pit initiation mechanism may be invoked. Most mechanisms involve a breakdown of the passive layer on a metal. The passive layer is thought to be a complicated layer on the surface of a metal. It is a layer which is 30 to 100 Angstroms thick. As an atom is only about 2 Angstroms in diameter, then a passive layer is only about 15 to 50 atoms thick. Experimentally this is very difficult to examine, especially in pitting investigations when the experimentalist does not know which site is going to pit. The passive layer is thought to be a two phase type of structure with the side nearest the metal a crystalline phase while the layer nearest the solution side is thought to be an amorphous mixture of metal ions and hydroxyl ions. Initiation Mechanisms 1. Defect Theory. Early investigators suspected that defects in the film broke down. The film defects were related to metal defects such as grain boundaries or slip steps due to dislocations emerging form the surfaces. These sites would be local anodes and initiate breakdown as the film probably was not fully formed over these local anomalies. Unfortunately, although some materials show a relationship between pits and defects, it is not a general rule. 2. Chloride Ion Dissolution. Many metal chlorides are soluble in water. One theory used this fact to suggest that at the solution/passive layer interface the chloride ion replaced the hydroxyl ion to form a metal chloride that dissolved. Another chloride ion at the same location then dissolved some more of the passive layer until the bare metal was exposed. Further chloride ion dissolution would then form a pit into the metal. This mechanism predicts that once a pit forms it will continue to grow. Unfortunately, some pits cease to grow. Pit Growth From a mechanistic point of view, the growth of a pit can be regarded as similar to the corrosion process in a crevice, covered in the previous section. The exposed surface outside the growing pit is cathodically protected by supporting the reduction of oxygen to hydroxyl ion reaction:- O2 + 2H2O + 4e- -> 4(OH-) As this cathodically protects the region outside the pit, the metal dissolution region cannot spread laterally across the surface. In addition the large cathodic surface can maintain this reaction and form a large cathode to small anode ratio which will accelerate the anodic reaction. Within the pit, which is regarded as a small hemisphere at this stage, the metal dissolution reaction is taking place. This is the general anodic reaction of:- M ->Mz+ + ze- However, it is the only reaction within the pit and results in an electrical imbalance again which attracts negatively charge ions, usually chloride ions. The autocatalytic reaction to form hydrochloric acid in the pit is initiated and continues:- Mn+ + Cl- + zH2O ->M(OH)z + z(H+Cl-) Pitting, like crevice corrosion, is an autocatalytic reaction once it is started and the pH decreases while chloride ion concentration increases inside the pit.Procedure:-1.A stainless steel sample is taken and the surface of the sample is thoroughly polished. 2. It is dipped in a solution containing 1 N H2SO4 and 0.3 N NaCl for a designated time. 3. The surface is observed under the optical microscope and the total number of pits on the surface are calculated. 4. The procedure is repeated with a similar sample for a different time interval

Observations:-Length of the sample- 2.4 cm.Width of the sample- 0.3 cm.Area= 0.75 cm2. Time interval=5 seconds Total number of pits=122 Pit density = 163 pits/cm2 Time interval = 15 seconds Total number of pits- 187.Pit Density= 250 pits/cm2. Time interval= 20 seconds Total number of pits= 225 Pit density=300 pits/cm2 Time interval = 30 seconds Total number of pits = 240 Pit density=320 pits/cm

Precautions: 1. Care is taken to ensure that the electrodes do not touch the glass-walls as it might lead to erroneous results. 2. Care is taken to avoid any parallax error. 3. The pit density doesnt increase after a specific time interval as new pits stop forming and the initiated pits grow in depth.