Principles of Corrosion PDF

Chapter 2 Principles of corrosion ⚫ Metals occur in nature in the form of their compounds (ores). Metal ∴Metals are less stable than E their compounds....

Chapter 2 Principles of corrosion ⚫ Metals occur in nature in the form of their compounds (ores). Metal ∴Metals are less stable than E their compounds. Ore (Cpd)  Corrosion ⚫ Corrosion may be defined as destruction of a material due to the contact with its surrounding environment. ⚫ Types of Corrosion 1. Direct or Chemical Corrosion (Dry Corrosion). 2. Indirect or Electrochemical Corrosion (Wet Corrosion).  CLASSIFICATION OF CORROSION Chemical corrosion (dry corrosion) Electrochemical corrosion (wet corrosion) It is a direct chemical reaction It is an electrochemical reaction between occurs between metal or alloy metals and the environment. surface and environment Takes place at the anodes of the micro Takes place by direct combination of electrochemical corrosion cells formed on the metal and its environment. There is metal surface no transport of electric charge. ex: A C A C C A Dissolution of metals in contact with A C A A non-conducting organic media. C C Destruction of metals attacked at high A A temperatures by aggressive gases. A C C A C A The five essential components of the corrosion cell are: 1- Anodic zones (A) 2-Cathodic zones (C) 3-electrical contact between them 4-an (ionically) conducting solution 5- cathodic reactant Requirements for Electrochemical Corrosion: Electronic Path ANODE CATHODE Ionic – Current Path Cathodic Where Reaction Corrosion Occurs!!!! Electrochemical Cells Galvanic Electrolytic (Voltaic) Cell Cell using electricity to force a generating electricity by chemical reaction to occur using a spontaneous (one that is non-spontaneous, chemical reaction (one DG = + ve). with DG = - ve) The two electrodes are connected with an external source of potential.. Electrochemical cells Galvanic cell Chemical Electrical Electrolytic cell Energy Energy Galvanic corrosion cell concentration corrosion cell Corrosion cell Electrolytic corrosion cell I- Galvanic (Voltaic) Cell Observe the electrodes to see what Salt bridge – is occurring. KCl in agar Provides conduction between half-cells Zn Cu 1.0 M ZnSO4 1.0 M CuSO4 ⚫ Daniell Cell  The corrosion cell reactions ⚫ The anodic reaction is: M → M+z + ze ⚫ The most common cathodic reactions are: (I) O2 + 4H+ + 4e- → 2H2O eo = 1.23 V O2 rich acidic media OR (II) O2 + 2H2O + 4e- → 4OH- eo = 0.401 V O2 rich non acidic media OR (III) 2H+ + 2e- → H2 eo = 0.0 V O2 poor acidic media OR (IV) 2H2O + 2e- → H2 + 2OH- eo = -0.828 V O2 poor non acidic media  The Electrochemical Series 2H2O + 2e- → H2 + 2OH- -0.828 Corrosive environment Air, water, soil, gases, and acids may acts as corrosive environment. Types of corrosive environment: Air: ❑ Moist and humid air is more corrosive than dry air. ❑ Hot air is more corrosive than cold air. ❑ Polluted air is more corrosive than fresh air. Water: Hot water is more corrosive than cold water. Salty water is more corrosive than fresh water. Polluted water is more corrosive than pure water. Gases: Acidic gases (SOx, NOx, and H2S.) are more corrosive than Basic gases (NH3). Soil: Presence of water, salts, gases, stray current, pollutants, affects the corrosion rate greatly.  Acid Mineral Organic Fatty Acids: Strength Very strong Weak Very weak Acids are very corrosive Examples H2SO4, Acetic and Animals environment. HNO3, Formic acid and plant H3PO4, oils HCl…… Bases: Strong bases (sodium hydroxide, potassium hydroxide) are more corrosive than weak bases (ammonia solution).