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5.2.1 Galvanic Corrosion Occur when 2 different metals are electrically connected in the same electrolyte. The less active (more noble) metal corrodes slower and will be protected. The galvanic series will predict which metal will corrode. The galvanic series is similar to the “emf” but is for alloys in real environment.
Experimental corrosion (zinc and cuprum) :
i. A zinc electrode connected to a cuprum electrode and immerse in an electrolyte such as salt water, acid or alkaline ii. The cuprum acts as cathodic and the zinc as anodic iii. Zinc will be corrode caused by the electrochemical corrosion iv. Time to time the zinc will continue to corrode and became embrittle, fragile and weakening.
Factors affecting the severity galvanic corrosion are :
1. Size of exposed areas of the anodic metal relative to that of cathodic metal. i. Smaller cathode relative to anode will cause small increase in corrosion of anode. ii. Smaller anode will suffer severe corrosion.
5.2.2 Crevice Corrosion Crevice corrosion occurs at shielded areas that contain small volume of aqueous solution. Crevice can be a hole, a space between the surface and a poorly adherent coating. Principle : 1. Liquid entry but stagnant 2. Corrosion rate of crevice is higher than that on bulk (outside) 3. Crevice corrosion is initiated by changes in local chemistry within the crevice; i. Depletion of oxygen in the crevice ii. Depletion of inhibitor in the crevice
Oxygen concentration can develop when there is a difference in oxygen concentration on a moist surface of a metal that can be oxidized. Example : 1. a drop of water/ moisture on the surface 2. the oxygen concentration are lesser on the surface 3. the surface that low in oxygen concentration are cathodic 4. the surface that has higher oxygen concentration are anodic 5. because there is anodic and cathodic, the surface below the water drop are corroded (anodic) 6. the water drop act as electrolyte Usually occurs at a bad gasket pipe flange, under bolt head and connections that soaked in liquid.
5.2.3 Intergranular Corrosion Is a localised attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. It is occur when different potential between atoms at the grain-boundaries and create the boundaries of anode and cathode. It is usually starts from the surface and accelerates internally causing by bad internal structure.
5.2.4 Stress Corrosion Cracking (SCC) It is refers to cracking caused by the combined effects of tensile stress and specific corrosion environment acting on the metal. Usually occurs in alloys not in pure metals and in certain environment, examples : copper cracked in ammonia or aluminium alloy cracked in chloride solubility. The stress in the materials must has its compressive component and the presence of both stress and corrosion environment which causing the cracks to form and spread. The stress corrosion cracking usually occurs between crystals.
5.3 Corrosion Control Cathodic protection is the protection of a metal by connecting it to a sacrificial anode or by impressing a direct current voltage to make it a cathode. Anodic protection is the protection of a metal which forms a passive film by the application of an externally impressed anodic current. Example (steel hulls of ships adjacent to the bronze propellers) : i. steel is an anode and bronze is a cathode and both are in sea-water which act as electrolyte ii. the steel (hulls) will be corroded because of its anodic, so a more anodic material than steel and bronze is used as corrosion sacrificial which it is zinc iii. zinc blocks are fitted to hulls so that the electrochemical corrosion process will occur only to the zinc iv. the zinc blocks must be replace time to time because its worn out of corrosion as shown below
5.4 Material Selection There are few combination between metal and good corroded environment and economical are shown below : i. stainless steel – nitrite acid ii. nickel and alloy nickel – caustic iii. monel – hydrofluoric acid iv. hastelloi (chlorimet) – hot hydrochloric acid v. plumbum – liquidifysulphuric acid vi. aluminium – unpolluted atmosphere exposion vii. tin – distillation water viii. titanium – hot oxidation liquid ix. tantalum – definite resistant x. steel – sulphuric acid
5.5 Coating Plastic and oil are non metal material use mainly for coatings. Metallic coatings which differ from the metal to be protected are applied as thin coatings to separate the corrosive environment from the metal. Metal coatings are sometimes applied so that they can serve as sacrificial anodes which can corrode instead of the underlying metal. Metallic coatings : 1. Noble coating it is a coating where higher potential electrode compared to the base metal will be protected. base metal coating such as cuprum, nickel and chromium as the coating and entering the holes in material. it cannot protect the base metal if there is holes in the coating because the base metal will become anode
2. Sacrificial coating the base metal protected by sacrifice it and the coated acts as anode the organic and inorganic material are used to protect the surface from contacting with oxygen or giving the basic protection by coated with stable material which cannot be penetrated by humidity/ moisture organic coating such as paint, tar, oil and varnish inorganic coating is enamel, plastic. Plastic is the main inorganic materials used as coating by hot dipping and spraying of corrosion resistant material
5.6 Design Designing rules : 1. considering corrosion penetration with the need of mechanical strength when determining the thickness of a metal used. It is important for piping and tank with liquid contents 2. welding is better than riveting for contena to reduce crevice corrosion. 3. use one type of material only for the whole structure to prevent galvanic corrosion. 4. avoid extra stress and stress concentration in corroded environment to prevent from crack-stress corrosion. Sharp edges of component need to be avoided because it can caused the stress 5. designing simple attachable system or changeable component if predicted it is easier to break or fail in the service
5.7 Painting Paint the surface of metal to avoid corroded material from contacting the surface. Paint may be applied by brushing, spraying and dipping. It may be dried naturally or by stoving. 5.8 Electroplate Metal Electroplating is the process of using electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material such as a metal using electrolysis. Electroplating and metal finishing processes include copper plating, nickel plating, zinc plating, silver plating, tin plating, brass plating, cadmium and chrome finishes. Metals plated include brass, copper, bronze, chrome, nickel, and black nickel, silver and gold.
The process : i. the metal/ components to be plated are immersed in a solution called electrolyte ii. electrolyte allows the passage of an electric current iii. the parts that require coating, are then placed in the solution and given a negative charge/ terminal (as cathode) iv. anodes are connected to the positive terminal v. upon the passage of an electric current metal ions are transferred from the electrolyte onto the surface of the cathode Electroplating allows for increased corrosion resistance, scratch resistance, decorative finishes and high temperature protection. Examples : tin plating and tin alloys for food container and food contact applications.
5.9 Oxide Layers Oxide layers such as zinc oxide and aluminium. It is higher in density and therefore preventing the oxygen and water from corrode the metal. The oxide layers also used as electroplating for metal products. Example : zinc oxide layers for steel roofs manufacturing. 5.10 Alloys A metal alloy is a combination of two or more metals or a metal and a nonmetal. Alloys are made to improved corrosion resistance. Steels usually alloyed with chromium and manganese to gain stainless steel.
5.11 Non-Ferrous Metal Metals and alloys are commonly divided into these classes : 1. ferrous metals : that contain a large percentage of iron 2. non-ferrous metals : that does not contain iron or only a relatively small amount of iron 3. a metal alloy : is a combination of two or more metals or a metal and a nonmetal Common non-ferrous metals used in engineering are : Aluminium b) Silver (Argentum)c) Copper (Cuprum) d) Plumbum/ Leade) Tin (Stanum)f) Nickel g) Zinc h) Chromium i) Gold (Aurum) j) Molybdenum k) Magnesium l) Cobalt m) Manganese Main properties of non-ferrous metals : 1. low strenght 2. good thermal and electric conductivity 3. free from magnetic field 4. high corrosion resistance 5. easier in manufacturing
5.11.1 Types of Non-Ferrous Metals, Physical Properties, Mechanic Properties and the Applications Refer table given: