1 / 15

Corrosion II

Corrosion II. Oxidation- Protective Oxide Films. Oxides form on metals due to reaction with air . Degree to which oxide films form depends on following factors. Volume ratio of oxide to metal consumed after oxidation should be close to 1. Good adherence.

libitha
Download Presentation

Corrosion II

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Corrosion II

  2. Oxidation- Protective Oxide Films • Oxides form on metals due to reaction with air. • Degree to which oxide films form depends on following factors. • Volume ratio of oxide to metal consumed after oxidation should be close to 1. • Good adherence. • High melting point of the film. • Low oxide pressure. • Coefficient of expansion equal to that of metal. • High temperature plasticity. • Low conductivity and diffusion coefficients of metal ions and oxygen. 13-27

  3. Oxidation- Protective Oxide Films The calculation of the volume ratio of oxide to metal after oxidation is a first step that can be taken to find out if an oxide of a metal might be protective. This ratio is called Pilling-Bedworth (P.B.) ratio. If a metal has a P.B. ratio of less than 1, the metal oxide will be porous, and unprotective. If the ratio is more than 1, compressive stress will be present and the oxide will tend to Crack and spall off.

  4. Given the atomic mass (g/mol) for W=183.86, O=16, Na=22.9, Hf =178.5

  5. Mechanisms of Oxidation • Oxidation partial reaction: M M 2+ + 2e- • Reduction partial reaction: ½ O2 + 2e- O2- • Oxidation starts by lateral expansion of discrete oxide nuclei. • Metal diffuses as electrons or cations across oxide films. • Sometimes O2- ions diffuse to oxide metal interface and electrons diffuse to oxide gas interface. 13-28

  6. Corrosion Control

  7. Material Selection Metallic Materials: • For reducing or nonoxidizing conditions such as air-free acid, nickel and copper alloys are often used. • For oxidizing condition, chromium –containing alloys are used. • For extremely oxidizing condition, titanium is often used.

  8. Material Selection

  9. Material Selection Nonmetallic Materials: • Polymeric materials such as plastic and rubber are less resistant to strong inorganic acids. • Ceramic have excellent corrosion and high temperature resistance. However they are brittle.

  10. Coatings • Metallic Coatings • Inorganic Coatings • Organic Coatings

  11. Design • Avoid excessive stress concentrations in corrosive environments to prevent stress -corrosion cracking. • Use weld rather than rivet containers to reduce crevice corrosion. • Design for easy draining and cleaning. • Avoid sharp bends in piping where flows occur. This is to prevent erosion corrosion caused by fluid direction change. • Deign systems for easy removal and replacement of parts that fail rapidly.

  12. Alternation of Environment • Lowering temperature • Decreasing the velocity of liquids • Removing oxygen from liquids • Reducing ion concentration • Adding inhibitors to electrolytes

More Related