🗊Презентация Corrosion

Steel Corrosion

Continual Corrosion

Local corrosion

Causes of localized corrosion

1. Oxidation corrosion 1. Oxidation corrosion (Reaction with oxygen) Some of the metals directly react with oxygen in the absence of moisture: Alkali and alkaline earth metals react with oxygen at room temperature and form corresponding oxides, while some metals react with oxygen at higher temperature. Metals like Ag, Au and Pt are not oxidized as they are noble metals.

Oxides of Pb, Al and Sn are stable and hence inhibit further corrosion. They form a stable, tightly adhering oxide film. Oxides of Pb, Al and Sn are stable and hence inhibit further corrosion. They form a stable, tightly adhering oxide film. In case of porous oxide film, atmospheric gases pass through the pores and react with the metal and the process of corrosion continues to occur till the entire metal is converted into oxide. Porous oxide layer is formed by alkali and alkaline earth metals. Molybdenum forms a volatile oxide film of MoO3 which accelerates corrosion. Au, Ag, Pt form unstable oxide layer which decomposes soon after the formation, thereby preventing further corrosion.

2. Corrosion by other gases (Cl2, SO2, H2S, NOx) In dry atmosphere, these gases react with metal and form corrosion products which may be protective or non-protective. Dry Cl2 reacts with Ag and forms AgCl which is a protective layer, while SnCl4 is volatile. In petroleum industries at high temperatures, H2S attacks steel forming FeS scale which is porous and interferes with normal operations.

3. Liquid - metal corrosion In several industries, molten metal passes through metallic pipes and causes corrosion due to dissolution or due to internal penetration. For example: liquid metal mercury dissolves most metals by forming amalgams, thereby corroding them. In oil  In sulphur  In organic substances

Factors Influencing Corrosion The nature and extent of corrosion depend on the metal and the environment. The important factors which may influence the corrosion process are: Nature of the metal (position Me in ECS) Environment (oxidant type) Concentration of electrolyte Temperature Hydrogen over voltage (pH)

Group 1 - alkali metals - the lowest corrosion resistance Group 1 - alkali metals - the lowest corrosion resistance 1 group of the sub-groups - very resistant metals Group 2 – are unstable, Group 2 sub-groups – are more stable (in the presence of oxygen to form a strong oxide film protecting from further destruction)

Group 3 - Aluminium - forming stable oxide film (but it is destroyed in solutions of acids and bases). In concentrated nitric and sulfuric acids passivated aluminum. Group 3 - Aluminium - forming stable oxide film (but it is destroyed in solutions of acids and bases). In concentrated nitric and sulfuric acids passivated aluminum. Group 4 – tin (Sn) and lead (Pb) – corrosion-resistant metals, thanks to strong oxide film.

5,6,7,8 group - by metals of subgroups have high ability to passivate and hence high resistance to corrosion. 5,6,7,8 group - by metals of subgroups have high ability to passivate and hence high resistance to corrosion. Osmium, iridium, platinum - the most resistant to corrosion Iron passivated by concentrated sulfuric and nitric acids

PROTECTION METHODS Material selection Improvements in material Design of structures Alteration of environment Add inhibitors Galvanic Cathodic or Anodic protection Active electrochemical protection Coatings