Corrosion Inhibition of Mild Steel

• The study investigates corrosion inhibition in mild steel within a saline solution containing zinc ions (Zn2+) and imidazole.
• The study was submitted to the University of Calicut for a Bachelor of Science in Chemistry degree.
• The research was conducted at N.S.S. College, Nemmara, under the guidance of Dr. Priya S.
• The project report certifies that the corrosion-inhibition study was carried out by 6th semester B.Sc. Chemistry students under the supervision of Dr. Priya S.
• The project report declares that the work is a genuine record of research done under guidance from Dr S. Priya S.
• Corrosion is the deterioration of materials through chemical interaction with their environment, generally referring to the degradation of metals. Modern corrosion science began early in the twentieth century, studying metallic corrosion since the Industrial Revolution.
• Electrochemical theory describes metallic corrosion as a coupled electrochemical reaction involving anodic metal oxidation and cathodic oxidant reduction, applicable in both wet and dry oxidation at various temperatures.
• The consequences of corrosion depend on exposure, application, and long-term performance, significantly affecting the safe operation of equipment and structures.
• Metals obtained from ores possess high energy and tend to revert to lower energy states, resulting in corrosion. Corroded metal loses mechanical strength and becomes unusable.
• Corrosion reactions are electrochemical; iron dissolves as ferrous ions at anodic sites, releasing electrons.
• Electrons pass to cathodic sites, reacting with reducible electrolyte components. Anodic and cathodic reaction rates are equivalent, determined by the corrosion current. Electrolyte conductivity influences corrosion cell operation.
• The corroding metal is a "mixed electrode" with simultaneous anodic and cathodic reactions.
• The most common electrochemical reactions in iron corrosion include:
  • Anodic reaction (corrosion): Fe → Fe2+ + 2e-
  • Cathodic reactions (simplified): 2H+ + 2e- → H2 or H2O + O2 + 2e- → 2OH-
• Oxygen reduction is most important in pH range 6.5–8.5. This is usually accompanied by solid corrosion debris.
• Pure iron (II) hydroxide is white, but the initial corrosion product is greenish due to partial oxidation. Further reactions can result in reddish rust which is a complex mixture of trace elements.
• Rust encourages further corrosion due to its porous and absorbent nature.
• A protective surface film retards further corrosion.

Factors Controlling Corrosion Rate

• Establishing anodic/cathodic locations concentrates damage; metals with differing electrochemical properties create a galvanic couple.
• Galvanic effects can be predicted by studying Galvanic Series, which list metals and alloys potentials in a corrosive environment, like seawater. More noble metals extract electrons from more negative ones.
• Aggressive ions like chloride prevent protective oxide film formation on metal surfaces and increase corrosion, NaCl is used winter de-icing and marine environments.
• Sulphur dioxide from fuel combustion dissolves in moisture, creates an acidic electrolyte stimulating anodic/cathodic reactions.
• Weight loss per unit area per unit time [mdd - milligrams per square decimeter per day].
• Rate of penetration [mpy - mils per year]
• Surface condition impacts rate of corrosion
• pH range of 4-10 can impact the rate of corrosion
• Oxygen content increase usually increases corrosion
• Flow rate increase can increase the rate of corrosion
• Water type can impact corrosion rates

Corrosion Inhibitors

• A known method of corrosion protection is using a corrosion inhibitor
• These methods are popular in industry due to low cost and easy practice
• Inhibitors are materials or a combination of materials used in small concentration within aggressive environments prevent or minimize corrosion. Mechanism of inhibitor is one or a number of three means:
  • Inhibitor chemically absorbs over time [chemisorption], creating protective film over metal
  • Inhibitor leads to creation of film to protect base metal
  • Inhibitor reacts to corrosion present within environments and creates a complex
• There are environmentally friendly corrosion inhibitors called "green" corrosion inhibitors.
• Chemicals can be synthetic or natural for corrosion inhibition.
• Chemical nature is organic or inorganic
• It can perform as a stand alone mix The present work identified methylene blue and poly vinyl pyrrolidone as corrosion inhibitors in mitigating corrosion rate of mild steel coupons in 3%, 5% and 10% sodium chloride solution.

Conditioning the Metal

• Can be broken down into 2 sub-groups:
  • Coating the metal creates a layer of corrosion resistance
  • Alloying can produce more resistant alloys like chromium or nickel

Conditioning the Corrosive Environment:

• Oxygen can be removed from water systems where pH is 6.5-8.5
• The oxygen is a component that is required for corrosion and the removal of it could be done through usage of sulphite.
• This method is not useful for open evaporative systems due to the constant access of oxygen in the atmosphere

Corrosion Inhibitors

• Chemical additives can reduce rate of metal wastage within corrosive environments
• Anodic Inhibitors interfere with the anodic process. If an anodic inhibitor isn't around the inhibitor can create pitting corrosion
• Cathodic Inhibitors reduce the oxygen in major cathodic reactions in cooling systems
• Adsorption-type corrosion Inhibitors use organic inhibitors and works through adsorption

Real-life Consequences of Corrosion:

• Titanic Disaster (1912): Corrosion of the ship’s metal hull contributed to its vulnerability, resulting in over 1,500 deaths.
• Exxon Valdez Oil Spill (1989): Corrosion of pipelines led to environmental damage and marine ecosystem costs.
• Piper Alpha Oil Rig Disaster (1988): Corroded safety equipment led to a catastrophic explosion, causing 167 deaths and major safety reforms.
• Silver Bridge Collapse (1967): A corroded eye-bar caused the bridge to collapse, resulting in 46 deaths and better inspection protocols.
• Flint Water Crisis (2014): Lead pipes caused lead contamination, resulting in health issues for thousands.
• Challenger Space Shuttle Disaster (1986): Corrosion in O-rings led to the failure of the rocket booster, costing 7 crew member's lives
• Mianus River Bridge Collapse (1983): Corroded supports led to the bridge collapse, killing 3 and improving inspections.
• Global Shipping Industry: Corrosion of ships leads to billions in maintenance costs
• San Onofre Nuclear Plant (2010s): Stress corrosion cracking led to plant shutdown and billions of dollars in costs needed for maintenance.

Electrochemical Control

• Corrosion studied by measuring changes in metal potential with time with applied electrical currents.
• Corrosion is controlled by passing anodic or cathodic currents into the metal
• Cathodic protection is utilized by passing electrons through suitable material to protect metal
• Protection can be achieved lowering the potential of the metal surface that is being protected by 300 or 400 mV
• Anodic dissolution of metal uses materials like aluminum or magnesium that are low in galvanic series

Corrosion Map of India:

• The map was prepared by the Corrosion Advisory Bureau, Metals Research Committee, Jamshedpur.
• It describes corrosion patterns in various regions, categorizing them as severe, most severe, and mild environments.
• Weight loss analysis is simplest method to test plant/equipment.
• Follows ASTM protocol. Involves weighing metal samples, exposing them, cleaning of the coupons, and reweighing.
• Weight loss is converted into a rate of loss (CR - corrosion rate/ ML - metal loss)
• Formula is Corrosion Rate = Weight loss x Constant / Alloy Density x Area exposed x Exposure time
• The technique requires a coupon and a coupon holder
• It is the oldest and most used method for corrosion loss

Coupon preparation:

• Important that testing is done before/after to assure accurate data.
• Surface cleaning includes bead,sand,stone, and grinding tools.
• Coupons of 25.8cm2 were cut for testing
• Mild steel was dried/weighed before performing tests with sodium chloride

Composition Of Well Water

• Water requisite was added to make NaCl 3% to simulate sea water
• Composition of ions were studied
• Mild steel was studied with 3% NaCl, with zinc, imidazole

Results Weight Loss Analysis

• Steel samples were exposed to sodium chloride for 25 days
• Samples cleaned and weighed again
• Density: 7.87 g/cm3 Area exposed: 25.8 g/cm2
• Weight loss rate in all different types of exposure:
  • NaCl(alone) - 1.446 mills
  • NaCl +1g Zn2+ - 1.3149 mills
• NaCl +2g Zn2+ - 0.8766 mills
• NaCl+3g Zn2+ - 0.4383 mills
• NaCl +2g Imidazole - 1.095 mills
• NaCl+2g Imidazole+2g Zn2+ - 0.4383 mills
• NaCl +3 ml Leaf Extract - 0.8766 mills
• As zinc ions increase weight loss and corrosion decreases, even with corrosive chloride
• 3% NaCl is higher due to chloride and destruction of ions
• Zn layer created provides protection in presence of Zn2+ ions
• Electrodes study confirms coatings with zinc.
• Zinc electrochemical potential is -0.76 V
• Iron is -0.44V
• Protective barrier formed when mild steel exposed
• Exposure rate decreased with rate
• Mild steel dipped spinach showed results to tested coupons
• Mild steel experienced less rate of corrosion with imidazole- Nitrogen atoms bind to metals and create protection,
• Zinc complex and Imidazole exposure significantly less corrosion- effective corrosion complex
• Bubble formations found on steel, surface analyzed under microscope
• Electro Chemical: open circuit potent measurement-
• OCP for steel immersed with concentrations of solution monitored.
• Shift shown as function of time.
• 3% NacL shifted towards Catholic region.
• Zn2+ ions increased. OCP shifted from the Anaodic regions. Ions form layer barrier for Electrons
• Coupes showed catholic potential values
• Shift had shift of anodic potential- Zin-imidazole showed shift towards cathodic.. alignment correlated to weight loss Additional comparison the potential shift of zinc show little relative to electrodes in NAcl.
• Mild stell- different weight loss in zinc
• Loss of study concentration of zing can impact corrosion- study of corrosion due to synergetic effect