Lecture - 40 Laboratory Experiments in Corrosion Engineering II Keywords: Polarization Experiments, Pitting Potentials, Microbial Corrosion. A. Electrochemical tests in a given environment Polarization curves and Tafel plots for generalized corrosion. Polarization resistance measurements. Corrosion potential, pitting and repassivation potential. Galvanic coupling effect on polarization curves. Electrochemical impedance (to study behavior of coating, passivation). Anodic polarization for establishing active-passive behavior of metals and alloys in different environments Anodic protection. Impressed current cathodic protection. In order to establish Tafel constants, corrosion potential, corrosion current and exchange currents, extrapolated regions of anodic and cathodic curves have to be used. Two types of approach a. Wide range of overpotentials with reference to rest potential (for example, -150mV to +150mV), to facilitate determination of Tafel slopes through extrapolation to the corrosion potential. b. Narrow range of overpotentials (+20 mv to -20mV), facilitating determination of linear polarization resistance (slow scan rate). 1
Description of cell and experimental arrangements are given in Fig 40.1 and 40.2. Fig. 40.1 Typical electrolytic cell with various electrodes for polarization measurements Fig. 40.2 Automatic polarization measurement 2
B. Measurement of pitting potentials: Cyclic polarization techniques can be used to evaluate pitting tendency of an active-passive metal or alloy (Fig. 40.3). A potential scan starting from E corr in the anodic direction is applied till significant current increase occurs. The final potential is negative with reference to repassivation potential. The potential where the loop closes on the reverse scan is the protection potential, which can also be estimated by extrapolating the reverse scan to zero current. Pitting potential (E pit ) corresponds to the potential at which current increases sharply. The larger the loop, the higher the tendency for pitting. Pitting shows up as an increasing anodic current before transpassive corrosion or evolution of oxygen. Fig. 40.3 Cyclic polarization to determine pitting and protection potentials. New pits can initiate only above pitting potential, and not between E pit and E prot. No hysteresis is exhibited by an alloy which is resistant to pitting. There will be potential and current distributions around pits. 3
C. Experiments for evaluation of sensitization in stainless steels. Study chromium depletion and precipitation as carbides at the grain boundaries. Oxalic acid test: Polished specimen is anodically etched at 1A/cm 2 for a minute in 10% oxalic acid at room temperature. Examine the specimen under the microscope to reveal step, dual or ditch structures. D. Determination of effect of alloy chemistry on passivation parameters: For development of corrosion resistant alloys with reference to active passive behavior, the following key parameters need to be optimized. E pp Primary passive potential i crit Critical anodic passivating current density. 1. Establish Anodic polarization curves for iron, nickel and chromium in 1N H 2 SO 4. Comment on the passivity curves with respect to passivity potential range, E pp and i crit. 2. Establish the effect of chromium (0 30%) in stainless steels on E pp and i crit in 1N H 2 SO 4. Plot your results with respect to (a) E pp Vs percent chromium (b) i crit Vs percent chromium 3. Determine pitting potentials for 18-8 stainless steel in different chloride concentrations. E. Corrosion testing for metals and alloys The following systems for corrosion testing are available, each covering different method of corrosion evaluation Humidity test chamber Salt spray Temperature and humidity control. Test set-up for alternate immersion testing of metals and alloys in 3.5% NaCl solution for stress corrosion testing. 4
Bimetallic corrosion testing in specific liquids under humidity control current and potential difference recorded. F. Laboratory techniques for studying amenability to MIC due to biofilms: Evaluation of biofilms from deposit samples collected from various locations organic and inorganic content of biofilms. Carbohydrate and protein analysis (spectrophotometer) Presence of aerobes and anaerobes in the deposits. Redox potential measurements in liquid samples. Corrosion potential measurements Biofilm growth on metal surface influences anodic and cathodic reactions Shifting of corrosion potentials in positive or negative directions to be monitored. Examples: Stainless steels in aerated seawater Mild steel in anaerobic seawater. Distinguish between aerobic and anaerobic corrosion. Polarization experiments in the presence and absence of biofilms on metals in the presence and absence of microorganisms. G. Monitoring and characterization procedures for different bacteria involved in MIC are listed in Table 40.1. Microbiological aspects of MIC microbes are illustrated in lectures 24 27. Various strains of different bacterial species can be procured from culture banks and characterized as per recommended procedures. 5
Table 40.1 Testing and Analyses of different bacteria relevant to MIC, Type of Bacteria Monitoring and characterization 1. Acid producing Production of inorganic and organic acids attack on various metals and alloys. 2. Denitrifying Ammonia production attack on copper alloys. 3. Iron-related Ferrous oxidation and ferric-reductionplugging of water and oil pipelines- tubercle formation. 4. Slime-forming Slime-sludge characterization. 5. Sulfate-reducing Sulfide production (H 2 S) FeS production - corrosion of metal surfaces. Common media used for routine isolation of bacteria and fungi Filamentous fungi - Potato Dextrose Agar Aerobic and anaerobic bacteria - Nutrient Agar Pseudomonas Sp - Select media from literature Sulfate Reducing Bacteria (SRB) - Postgate media 6
H. Microbially influenced corrosion of aluminium alloys Choose desired aluminium alloys and make suitable specimens. Use naturally collected sea water and fresh water samples from identified locations. A biofilm growth chamber (under conditions of both stagnant and flowing liquid) can be constructed to expose the metallic specimens for different periods of time. Monitor biofilm growth by removing specimen frequently and characterize the biofilm with respect to thickness, microbial assay, chemical and metallurgical analysis, surface roughness and morphology. Isolate important bacterial species from the biofilm and carry out steady-state potential and polarization measurements in the presence and absence of isolated bacteria. I. Biofouling and MIC of stainless steels in sea water. Experiments similar to the previous one for aluminium alloys. J. Microbial diversity of pipelines and establishment of MIC Locate a pipeline transporting water and petroleum products. From the pipeline, collect aseptically, samples of water, oil and corrosion products (debris). a) Visual, physical and chemical characterization of the water, oil and debris samples for colour, chemical composition, ph. b) Isolation and enumeration of different types of microorganisms through standard microbiological procedures characterization of isolated organisms with respect to Autotrophs, Heterotrophs. Bacteria, fungi Aerobe, anaerobe Iron oxidising, Manganese oxidising. c) Based on the microbial assay and characterization and failure analysis of the pipeline samples, predict nature of MIC (Microbially influenced corrosion) 7
K. Examination of biocorrosion of concrete in the laboratory simulating sewer conditions Samples of sewer pipes collected from sewerage processing stations can also be used. Corrosion testing coupons Fresh coupons from new sewer pipes. Coupons prepared from corroded concrete slabs from sewer treatment plants. Corrosion chambers for exposing the coupons to bacterial activity under simulated conditions can be used. Monitor conditions with respect to ph changes, H 2 S generation, temperature and humidity. Growth of Anaerobic Sulfate Reducing Bactria (SRB), aerobic sulfur oxidizers such as Acidithiobaullus can be monitored and their role on concrete corrosion assessed. L. Bacterial kinetics of sulfur oxidation of Acidithiobacillus thiooxidans and its influence on concrete corrosion. Experimental strategy: Bacterial growth in recommended media. Growth curve with respect to cell number, ph and sulfate concentration as a function of time. Establish bacterial growth kinetics. Concrete corrosion tests in aqueous media at bacterial acidic ph under different conditions of temperature, metal-ion concentrations, and types of reinforcement steels. 8
M. Corrosion testing for medical implants Compatibility Tissue response Dissolution rates Toxicity In vivo corrosion How susceptible is the implant metal to corrosion? Effect of corrosion on body response Rest potential : Measurements over extended periods of time to predict metal dissolution. Cyclic potentiodynamic polarization: Corrosion susceptibility of small implant devices. Galvanic corrosion: Coupled and uncoupled leach rates. Fretting: Fretting corrosion in moving body parts. Various metal and alloy samples representative of implant materials can be shaped into electrodes and tested in body fluids and simulated electrolytes. 9