CORROSION OF METALS CAUSES OF DECAY CHEMICAL AND PHYSICAL AGENTS OF DETERIORATION WATER AND MOISTURE

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1 CORROSION OF METALS CAUSES OF DECAY CHEMICAL: Moisture, oxygen, soil, atmospheric contaminants PHYSICAL: Temperature, sunlight, wind, water BIOLOGICAL: Microorganisms, insects CHEMICAL AND PHYSICAL AGENTS OF DETERIORATION Gases (O 2, O 3, CO 2, H 2 O, SO 2, H 2 S) Liquids (Water) Solids (Smoke, dust particles, salts in soil) Energy available in wind Erosion of water flow Sunlight WATER AND MOISTURE It is a physico-chemical agent It washes away particles from a given body It has a strong erosive action (streams, rain, snow) It dissolves and decomposes particles Volume changes causes physical breakdown Acts as localized explosive 1

2 OXYGEN TEMPERATURE EFFECTS Ozone (O 3 ): -It is an allotrope of O 2 -More powerful oxidizing agent than O 2 -Moisture is not essential Oxygen (O 2 ):-Needs moisture to act as a corrosive agent Chemical: Physical: -For every 10 o C increase, chemical reaction rate doubles -Dimensional change on materials -Low thermal conductivity causes: a) distortion of shape b) cracking and breaking -Coefficient of thermal expansion causes physical breakdown EFFECTS OF SUNLIGHT Effects of short wave ultraviolet radiation a) Photochemical degradation (especially on organic substances) b) Disintegration of textiles c) Bleaching of dyestuffs d) Health effects SOIL CONTAMINANTS Acid/alkaline soils destroy original properties of materials Crystallization in the pores causes internal pressure Disintegration by chemical reactions 2

3 ATMOSPHERIC CONTAMINANTS METAL CYCLE SO 2 + H 2 O H 2 SO 3 H 2 SO3 + 1/2O 2 H 2 SO 4 (Sulfuric acid) NO 2 + H 2 O HNO 3 (Nitric acid) H 2 S (Hydrogen sulfide) forms a dark-colored corrosion layers commonly found in archaeological artifacts and old buildings Smelting Metal Ore Production Corrosion Metal Artifatcs OXIDATION POTENTIAL OF SOME METALS E O (volts) Au Au e Ag Ag + + e Cu Cu e Pb Pb e Sn Sn e Fe Fe e Ni Ni e Zn Zn e Al Al e Mg Mg e STABILITY OF METAL OXIDES OXIDE ΔG(kJ) Au 2 O Ag 2 O Cu 2 O Fe 2 O SnO

4 SPONTANEOUS REDUCTION OF SILVER (Ag + ) by COPPER METAL CORROSION = RUSTING Metal + Environment = Metal compound + Energy TARNISHING = Lost of metallic luster WHY DO METALS CORRODE? Metal + Environment Metal Compound + Energy There are two important concern about chemical Equation: 1. In which direction will the reaction proceed? 2. How long will it take for the reaction to reach equilibrium? The 2nd law of thermodynamics determines the spontaneous direction of chemical reaction which is governed by two parameters: 1. Enthalpy: H, the energy change 2. Entropy: S, order change Spontaneous reactions can produce work. The maximum amount of work is represented by the free energy ( G)( of the reaction and is related to H H and S S by the relationship G G = H H - T ST 4

5 Free Energy, G, can be measured experimentally and if: C (Diamond) C(Graphite) G G < 0, The reaction will be spontaneous as written. G G > 0, The reaction will be spontaneous in the reverse direction. G G = 0, The reaction is in a state of equilibrium. H H = -450 cal/mol S S = 0.79 cal/mol deg G G = H H - T ST = x 0.79 G G = -685 cal/mol BIMETALLIC CORROSION MECHANISM OF CORROSION 5

6 CHEMICAL MECHANISM OF CORROSION H 2 O H + + OH - (M) Metal + nh + M +n + nh 2H 2H + 1/2O 2 M +n + noh - H 2 (gas bubbles) H 2 O M(OH) n Corrosion Potentials of Metals in Seawater Metal E coor (v) Silver 0.26 (least active) Admiraly brass 0.10 Copper 0.07 Yellow brass 0.04 Lead Mild steel Tin lead solder Zinc (most active) GALVANIC PROTECTION GALVANIC PROTECTION When two metals with different E corr values are brought into electrical contact, electrons will flow from the metal with lower E corr value into that with higher E corr value. This will cause an increase in the corrosion rate of the piece with lower E corr value 6

7 FACTORS AFFECTING CORROSION 1. Moisture content 2. Porosity of the metal 3. The acidity of the burial site 4. The presence of soluble salts 5. The temperature Factors Affecting Metal Corrosion in Seawater Dissolved oxygen ph Temperature Water Movement Salinity Sulphate reducing bacteria Galvanic coupling Marine growth DISSOLVED OXYGEN IN WATER ACIDITY (ph) Temperature ( 0 C) Oxygen (ml/lt) ph = - log [H + ] Normal seawater has a ph range of (slightly basic) It is well buffered by the action of HCO 3- /CO -2 3 equilibrium 7

8 SALINITY Salinity of open ocean is about 35 g/liter Major ions in seawater are: Cl - = g/kg SO 4 = 2.79 g/kg HCO - 3 = g/kg 1. Provide an ionic conducting solution 2. Forms protective surface layers 3. Alters the amount of dissolved oxygen in water 4. Supply ions which can catalyze corrosion CORROSION PRODUCTS OF COPPER 1. Cuo (black) Tenorite: : forms when hot copper is exposed to air. 2. Cu 2 O (red) Cuprite: forms when copper reacts with air at ambient temperatures. 3. CuCO 3 Cu(OH) 2 (green-blue) Malachite or Azurite: forms in soil with carbonic acid. 4. Cu 2 S (black) Calcosite: : forms in polluted air by reacting with H 2 S. 5. CuFeS 2 : Chalcopyrite and Cu 5 FeS 4 : Bornite: forms in soil if sulphate reducing bacteria exists in iron- rich soil. 6. CuCl 2 3Cu(OH) 2 (dark green) Atacamite: : forms with NaCl or saline environment CORROSION PROCESS OF COPPER 1. Initially a brown film forms. 2. Several months later, crystals of CuSO 4 forms and the surface becomes black. 3. After 5 years some sulphide (CuS( CuS) ) and carbonates (CuCO 3 ) forms. 4. Eventually green patina appears CORROSION OF COPPER Anode: Cu Cu + + e - Cu Cu e - Cathode: O 2 + 2H 2 O + 4e - 4(OH) - 2H + + 2e - H 2 BTA, inhibits both the anodic and the cathodic processes. 8

9 CORROSION OF RONZE Since tin is more reactive than copper, in general bronze corrodes faster than copper. Corrosion is generally concentrated along the grain boundaries which are more reactive and anodic compared to the grains. Both stannic and stannous oxides form which yield patina and mah hinder further corrosion BRONZE DISEASE Occurs when bronze has been buried in moist chlorine containing soils. Appears as a pale green deposits on the surface of the objects. It will completely corrode and underlying metal. It is autocatalytic and HCl will recycle. 2CuCl + H 2 O CuO 2 + 2HCl 2HCl + Cu + 1/2O 2 2Cu + 1/2O 2 2CuCl + H 2 O Cu 2 O CORROSON OF IRON Anodic Reaction: Fe Fe 2 + 2e - Cathodic Reactions: 1. In deaerated solutions: 2H + + 2e - H 2 H 2 O + 2e - 2(OH) + H 2 2. In aerated solutions: O 2 + H 2 O + 4e - 4(OH) - + 2H + + 4e - 2(OH) - O 2 FORMATION OF RUST Fe (OH) - Fe(OH) 2 4Fe(OH) 2 + O 2 2Fe 2 O 2.H 2 O + H 2 O 9

10 CORRODED IRON CANON CORRODED ANCOR PADLOCK 10

11 CORROSION OF TIN Corrosion product of tin artifacts is cassiterite (SnO 2 ) which is not a passivating film. The general shape of the artifact is maintained but the matrix os often crumbly The temperature of marine sites are too high for the occurance of tin pest (the change in crystal structure from β tin to α tin. CORROSION OF LEAD Lead artifacts from land sites are often covered with cerrussite (PbCO 3 ). Some lead (II) and lead (IV) oxides as well as anglesite (PbSO 4 ) may also form. Anglesite and laurionite (Pb(OH)Cl) form in aerobic marine sites and PbS in anaerobic sites. Formation of insoluble anglesite protects the underlying metal and increases the reduction potential of the metal CORROSION OF SILVER AND ITS ALLOYS Although silver is a noble metal, in marine environment it corrodes to form cerargyrire (AgCl) and argentite (AgS 2 ). The corrosion products produces a negative shift in silver reduction potential, which dramatically increases its reactivity. Conservation of silver coins 11

12 ORGANIC INHIBITORS Polar organic compounds containing N, O and S can ionize to give positively charged ammonium oxonium or sulfonium ions which are absorbed at the cathode and reduce evolution oh hydrogen The inhibitive effect of these compounds depend on: 1. Chain length 2. Cross sectional area 3. Inclination of the molecule to the metal surface 4. Other stereochemical properties BENZOTRIAZOLE (BTA) Corrosion inhibitor for copper and copper alloys Used for over 30 years Mansen in 1967 employed it to copper artifacts Important in the storage of corroded artifacts and preservation of the stabilized antiquities C 6 H 5 N 3 is a white powder; m.p. = 95 o C Solubility in water at 100 o C is 20%; in isopropyl alcohol at 20 o C is 40% Exists as an anion in most solutions BENZOTRIAZOLE (BTA) N N H N APPLICATION OF BTA 1. The object is cleaned to remove loose corrosion and dirt. 2. It is degassed in a mixture of 50:50 acetone and toluene. 3. It is immersed in a 3 % (by weight) BTA solution in industrial methyleted spirits. 4. A vacuum is applied untill no more air bubble appears which ensures the complete penetration of solution 12

13 BTA Vapor from solid BTA inhibits corrosion of articles in a localized atmosphere Used as an ingredient in lacquers and waxes Used for stabilization of aggressive patinas PROVENTION OF BRONZE DISEASE 1. Keep the bronze in dry place (Relative humidity < 35%) 2. Remove all cuprous chloride. 3. Make the article cathodic in distilled water. 4. Treat active areas with silver oxide that selas with a coating of silver chloride. 5. The unstable cuprous chloride can be reduced by applying zinc filings and strong sulfuric acid 6. Use BTA to combine with cuprous chloride to form a stable copper BTA complex/ CONSERVATION OF BRONZE 13