Lecture:Metallurgical Aspect of Corrosion

Lecturer: Asst.Prof.Dr.Ali Sabea Hammood

University of Kufa- Faculty of Engineering-

Materials Engineering Dapartment

Structural features:

1- Crystal feature:

- Crystallographic arrangement for atoms in lattice.

2- Growth features:

- Grain boundary , sub-boundaries , lattice defects - Imperfections , surface atomic topography.

3- Deformation features:

- Slip markings , twin bands , stacking faults , dislocation arrays.

• ● Dislocation : can be occur as result of: heterogeneous character for chemical composition and metallurgical structure.

Etch pits:

Because strain field surrounded for dislocation

Sever chemical reactions

Darkened the structure

4- Microstructure :

- Impurities , precipitation-secondary phases which result from "phase transformation and compositional variations"

- When concentration changes in metallic materials

Segregation of solute and impurities of grain and dislocation-

solute interactions

Relationships between electrochemical and metallurgical aspect of corrosion:

1- Specific metallurgical heterogeneities:

Local cell action

• Structure sensitivity of corrosion reaction: Microstructure parameters : low effect in "uniform corrosion rates"

• High effect in "localized or preferential corrosion“

• Grain boundary:• "atomic mismatch region"• higher energy state and greater chemical

reactivity

Corrosion process consist of two stages:

1- Nucleation or initiation stage2- Growth and propagation stage

Nucleation or initiation stage: - Non-homogeneities (kinks , ledges , terrace , jogs , point defects) - Changing for perfection of surface films. - Impurities at the surface(situations: metallurgical non-homogenization and non- homogenization in chemical composition )

High intensity of chemical reactivity

Growth and propagation stage:

Kinetic factors: "film formation" _ "diffusion reaction " _ "polarization" 

• Corrosion Control by Application of Metallurgical principals:• It is possible to reduce or prevent corrosion by application of the following metallurgical principals:•  • 1-Use of high purity material.•  • 2-Use of alloying additions.•  • 3-Effective heat treatments..•  • 4-Use surface coatings.

•  

• A high purity:• One of the best examples of improvement of corrosion resistance of a materials found in results achieved by reducing the sulfur content of plain carbon steels. Corrosion attack on a steel greatly reduced when its sulfur level is low. Lead in zinc die cast alloys also has a marked effect on the corrosion characteristics of the material. If the lead exceeds 0.002%, it precipitates at the grain boundaries and produces an increased tendency for intergranular corrosion

 •  Stainless steel 300L has a maximum limit of 0.03% carbon. This reduces the possibility of sensitization as a result of welding or/and heat treatment.

Alloy additions:

It is possible to improve the corrosion resistance of certain alloys by specific alloying additions. The most notable is the addition of chromium to iron in amounts of 12% or more. At or above this concentration, a passive film o chromium-iron oxide is formed on the surfaces, which are the basics of stainless steel corrosion resistance. Corrosion resistance also may be improved by changing the electrochemical potential of the second phase in an alloy. For example, the addition of magnesium or chromium to aluminum alloys. In these alloys, the intermetallic compound, FeAl

, greatly affects pitting and corrosion resistance. The addition of magnesium or chromium changes the FeAl

• to complex Al-Fe-Mn or Al-Fe-Cr compound whose potential now approaches that of the aluminum itself. Consequently, the pitting tendency is greatly reduced.

• The intergranular  sensitization of stainless steels due to the formation of chromium carbide at grain boundaries can be prevented using alloying additions. In these alloys, it is possibleto add columbium or titanium which selectively ties up the carbon as carbides, eliminating chromium carbide formation and the sensitization of the alloys as well. The addition of chromium, particularly to nickel and iron base materials increases their high temperature oxidation resistance

• Heat treatment :• It has been discussed later that it is possible to modify the structure of metals and alloys in many ways through heat treatment. Age-hardening heat treatments also have great effect on the corrosion resistance of alloys. Many alloys have a temperature range for aging in which they are not susceptible to intergranular corrosion. However, the aging temperature for optimum mechanical properties of an alloy may reduce susceptibility to intergranular corrosion. Often it is important to sacrifice some mechanical properties in order to improve corrosion resistance.

•  It has been shown recently that stress relief greatly improves the resistance of 300 Series stainless steels to stress corrosion. Of course, the stress relieving must be below the temperature of sensitization, 427°C. High zinc brasses, susceptible to stress corrosion in the cold working state, also have been protected effectively by stress relief. Heat treatment of the surfaces to increase surface hardness of the material or improve the stability of surface films is important in resisting all types of corrosion but is particularly useful in improving fretting and erosion-corrosion resistance.

Surface coatings for corrosion control:• Because the corrosion reactions occur at metal-environment interface, it is logical that the interpositions of barriers between the substrate and the environment would influence the corrosion rate. Various types of barriers are commonly used for corrosion control including a wide range of metals, inorganic and organic materials.

References:•1-Zaki Ahmed, Principles of corrosion engineering

and corrosion control, First Edition, BH, 2006.•2 -M.G.Fontana& N.D.Greene, Corrosion

Engineering, Mc Graw-Hill, 2000 .•3-P.R.Roberge,Corrosion, Engineering, Principle&

Practice McGraw Hill,2008.•4-P.R.Treathewey and T.Chamberlian, Corrosion for

Science &Engineering,2nd Edition,Longman,1996 .5-Ali Sabea Hammood, The Microstructure and

Corrosion Resistance of Cast Cupronickel Alloys For Naval and Non-Naval Applications, PhD –Metallurgical Engineering, University of Technology,2003.