CORROSIVE DAMAGE IN MATERIALS & ITS PREVENTION Dr. T. K. G. NAMBOODHIRI Professor of Metallurgy (Retired)

INTRODUCTION Definition: Corrosion is the degeneration of materials by reaction with environment. Examples: Rusting of automobiles, buildings and bridges, Fogging of silverware, Patina formation on copper.

UNIVERSALITY OF CORROSION Not only metals, but non-metals like plastics, rubber, ceramics are also subject to environmental degradation Even living tissues in the human body are prone to environmental damage by free radicals-Oxidative stress- leading to degenerative diseases like cancer, cardio-vascular disease and diabetes.

CORROSION DAMAGE Disfiguration or loss of appearance Loss of material Maintenance cost Extractive metallurgy in reverse- Loss of precious minerals, power, water and man-power Loss in reliability & safety Plant shutdown, contamination of product etc

COST OF CORROSION Annual loss due to corrosion is estimated to be 3 to 5 % of GNP, about Rs.700000 crores Direct & Indirect losses Direct loss: Material cost, maintenance cost, over-design, use of costly material Indirect losses: Plant shutdown & loss of production, contamination of products, loss of valuable products due to leakage etc, liability in accidents

WHY DO METALS CORRODE? Any spontaneous reaction in the universe is associated with a lowering in the free energy of the system. i.e. a negative free energy change All metals except the noble metals have free energies greater than their compounds. So they tend to become their compounds through the process of corrosion

ELECTROCHEMICAL NATURE All metallic corrosion are electrochemical reactions i.e. metal is converted to its compound with a transfer of electrons The overall reaction may be split into oxidation (anodic) and reduction (cathodic) partial reactions Next slide shows the electrochemical reactions in the corrosion of Zn in hydrochloric acid

ELECTROCHEMICAL REACTIONS IN CORROSION

ELECTROCHEMICAL THEORY The anodic & cathodic reactions occur simultaneously at different parts of the metal. The electrode potentials of the two reactions converge to the corrosion potential by polarization

PASSIVATION Many metals like Cr, Ti, Al, Ni and Fe exhibit a reduction in their corrosion rate above certain critical potential. Formation of a protective, thin oxide film. Passivation is the reason for the excellent corrosion resistance of Al and S.S.

FORMS OF CORROSION Corrosion may be classified in different ways Wet / Aqueous corrosion & Dry Corrosion Room Temperature/ High Temperature Corrosion

WET & DRY CORROSION Wet / aqueous corrosion is the major form of corrosion which occurs at or near room temperature and in the presence of water Dry / gaseous corrosion is significant mainly at high temperatures

WET / AQUEOUS CORROSION Based on the appearance of the corroded metal, wet corrosion may be classified as Uniform or General Galvanic or Two-metal Crevice Pitting Dealloying Intergranular Velocity-assisted Environment-assisted cracking

UNIFORM CORROSION Corrosion over the entire exposed surface at a uniform rate. e.g.. Atmospheric corrosion. Maximum metal loss by this form. Not dangerous, rate can be measured in the laboratory.

GALVANIC CORROSION When two dissimilar metals are joined together and exposed, the more active of the two metals corrode faster and the nobler metal is protected. This excess corrosion is due to the galvanic current generated at the junction Fig. Al sheets covering underground Cu cables

CREVICE CORROSION Intensive localized corrosion within crevices & shielded areas on metal surfaces Small volumes of stagnant corrosive caused by holes, gaskets, surface deposits, lap joints

PITTING A form of extremely localized attack causing holes in the metal Most destructive form Autocatalytic nature Difficult to detect and measure Mechanism

DEALLOYING Alloys exposed to corrosives experience selective leaching out of the more active constituent. e.g. Dezincification of brass. Loss of structural stability and mechanical strength

INTERGRANULAR CORROSION The grain boundaries in metals are more active than the grains because of segregation of impurities and depletion of protective elements. So preferential attack along grain boundaries occurs. e.g. weld decay in stainless steels

VELOCITY ASSISTED CORROSION Fast moving corrosives cause a) Erosion-Corrosion, b) Impingement attack , and c) Cavitation damage in metals

CAVITATION DAMAGE Cavitation is a special case of Erosion-corrosion. In high velocity systems, local pressure reductions create water vapour bubbles which get attached to the metal surface and burst at increased pressure, causing metal damage

ENVIRONMENT ASSISTED CRACKING When a metal is subjected to a tensile stress and a corrosive medium, it may experience Environment Assisted Cracking. Four types: Stress Corrosion Cracking Hydrogen Embrittlement Liquid Metal Embrittlement Corrosion Fatigue

STRESS CORROSION CRACKING Static tensile stress and specific environments produce cracking Examples: 1) Stainless steels in hot chloride 2) Ti alloys in nitrogen tetroxide 3) Brass in ammonia

HYDROGEN EMBRITTLEMENT High strength materials stressed in presence of hydrogen crack at reduced stress levels. Hydrogen may be dissolved in the metal or present as a gas outside. Only ppm levels of H needed

LIQUID METAL EMBRITTLEMENT Certain metals like Al and stainless steels undergo brittle failure when stressed in contact with liquid metals like Hg, Zn, Sn, Pb Cd etc. Molten metal atoms penetrate the grain boundaries and fracture the metal Fig. Shows brittle IG fracture in Al alloy by Pb

CORROSION FATIGUE S-N DIAGRAM Synergistic action of corrosion & cyclic stress. Both crack nucleation and propagation are accelerated by corrodent and the S-N diagram is shifted to the left

CORROSION FATIGUE, CRACK PROPAGATION Crack propagation rate is increased by the corrosive action

PREVENTION OF CORROSION The huge annual loss due to corrosion is a national waste and should be minimized Materials already exist which, if properly used, can eliminate 80 % of corrosion loss Proper understanding of the basics of corrosion and incorporation in the initial design of metallic structures is essential

METHODS Material selection Improvements in material Design of structures Alteration of environment Cathodic & Anodic protection Coatings

MATERIAL SELECTION Most important method – select the appropriate metal or alloy . “Natural” metal-corrosive combinations like S. S.- Nitric acid, Ni & Ni alloys- Caustic Monel- HF, Hastelloys- Hot HCl Pb- Dil. Sulphuric acid, Sn- Distilled water Al- Atmosphere, Ti- hot oxidizers Ta- Ultimate resistance

IMPROVEMENTS OF MATERIALS Purification of metals- Al , Zr Alloying with metals for: Making more noble, e.g. Pt in Ti Passivating, e.g. Cr in steel Inhibiting, e.g. As & Sb in brass Scavenging, e.g. Ti & Nb in S.S Improving other properties

DESIGN OF STRUCTURES Avoid sharp corners Complete draining of vessels No water retention Avoid sudden changes in section Avoid contact between dissimilar metals Weld rather than rivet Easy replacement of vulnerable parts Avoid excessive mechanical stress

ALTERATION OF ENVIRONMENT Lower temperature and velocity Remove oxygen/oxidizers Change concentration Add Inhibitors Adsorption type, e.g. Organic amines, azoles H evolution poisons, e.g. As & Sb Scavengers, e.g. Sodium sulfite & hydrazine Oxidizers, e.g. Chromates, nitrates, ferric salts

CATHODIC & ANODIC PROTECTION Cathodic protection: Make the structure more cathodic by Use of sacrificial anodes Impressed currents Used extensively to protect marine structures, underground pipelines, water heaters and reinforcement bars in concrete Anodic protection: Make passivating metal structures more anodic by impressed potential. e.g. 316 s.s. pipe in sulfuric acid plants

COATINGS Most popular method of corrosion protection Coatings are of various types: Metallic Inorganic like glass, porcelain and concrete Organic, paints, varnishes and lacquers Many methods of coating: Electrodeposition Flame spraying Cladding Hot dipping Diffusion Vapour deposition Ion implantation Laser glazing

CONCLUSION Corrosion is a natural degenerative process affecting metals, nonmetals and even biological systems like the human body Corrosion of engineering materials lead to significant losses An understanding of the basic principles of corrosion and their application in the design and maintenance of engineering systems result in reducing losses considerably