Materials Performance Corrosion/SCC,EAC, etc. Review

Review Corrosion involves ? reactions. What is an Oxidation Reaction? LEO What is a Reduction Reaction? GER What will control the rate of a corrosion reaction? (2) What is the driving force for corrosion reactions? What does the Nernst Equation tell us? What is a Pourbaix Diagram? What is the basis for a Pourbaix Diagram? What are the 2 diagonal lines on the Pourbaix Diagram? What is stable in the region between those line?

Corrosion of zinc in acid Zn  Zn2+ + 2e- Electrochemical Potential 2H+ + 2e-  H2 Rate of Reaction

Review (2) What do we mean by “passivity”? How can we passivate an alloy or metal? What is special about a Fe-12 Cr alloy with respect to passivity? What is a polarization curve? What are the various regions on a polarization curve? What does SHE mean? SCE?

Pourbaix Diagram for Iron Will iron corrode in acid? 2.0 1.6 Fe oxides stable 1.2 O2 + 4H+ + 4e- = 2H2O Fe3+ 0.8 Potential 0.4 Yes - there is a reasonably wide range of potentials where H2 can be evolved and iron dissolved 0.0 2H+ + 2e- = H2 -0.4 Fe2+ stable -0.8 -1.2 Fe metal stable -1.6 7 14 pH

What is Passivity? Formation of an oxide/compound on a metal or alloy surface that is stable in the electrolyte, so that the metal is rendered “passive” in the environment (i.e., the material is “passivated”) Passivation limits corrosion Generally, strong oxidising conditions are required for passivation Note: Passive films have some electrical conduction

More on Passivity... Many alloys exhibit passivity, such as Fe-Cr alloys Fe-Cr alloys exhibit an increasing tendency to passivate as the Cr content increases! The critical current density required for passivity in deaerated neutral solutions decreases as the Cr content is increased to 12 wt.%, beyond which it is constant Fe-12+%Cr alloys and stainless steels are self-passivating (no externally applied current nor strongly oxidizing conditions are required for passivation)

Review: Polarization Curve M  M+n + ne- Pitting Passive Potential (V) (SHE) ANODIC Ecorr Active CATHODIC Log (current density)

Minimising Weld Decay “Weld decay” is the preferential corrosion of the region within the heat-affected zone (HAZ) of austenitic stainless steel welds How can we ‘solve’ a weld decay problem? Use a low C grade of austenitic stainless steel (304L) Use a stabilised austenitic stainless steel (347, 348): Nb and Ti additions to promote high T carbide formation, thereby minimising Cr-rich carbide precipitation Control of weld heat input/welding parameters

Review (4) Wht 3 factors contribute to localised corrosion? What are the types of localised corrosion? Define. What factors are required for galvanic corrosion? How can galvanic corrosion be influenced? What are the differences between the standard EMF series and the Galvanic series? Differential aeration is a form of localised corrosion due to ? Crevice corrosion requires ?

Galvanic Corrosion

Why?

Review (5) The type of corrosion that occur due to local breakdown of a passive film on an open surface is ? Dealloying, also known as ?, requires an alloy with one component more ? than the other. One component corrodes ?. What makes austenitic stainless steel susceptible to intergranular corrosion? The presence of intergranular Cr-rich carbides ? the alloy sensitive to preferential intergranular corrosion. What Cr level is required for passivity of austenitic stainless steels?

Review (6) Weld decay can occur where? Why? Weld decay can be minimised/avoided by? Flow can ? corrosion. Erosion corrosion involves ? How can you prevent corrosion? Name and describe 3 methods.

Review (7) What is SCC? What is required for SCC? Give an example of SCC. Explain. What are some mechanisms for SCC? How do they explain SCC? What type of tests are used to assess material susceptibility to SCC What parameters are used to characterise SCC? (How can we measure SCC?) What is Hydrogen Embrittlement? What is HELP? How does one determine whether a material has failed by HELP? What is Hydrogen Attack (HA)? In what type of environment or industry might you encounter HA?

Review (8) If a material is susceptible to hydrogen embrittlement, why can’t we use cathodic protection to prevent SCC? What is Corrosion Fatigue? Name two mechanisms for Corrosion Fatigue? How can we minimise or eliminate SCC? Name 3 things that can be done. How can Hydrogen Embrittlement be a mechanism of SCC? One mechanism for SCC is called the Film Rupture or Slip/Dissolution mechanism. This type of mechanism is a type of ?

Review (9) What is KISCC? What type of specimen is required to perform an SCC Crack Growth Rate test? What can we determine from a SSR Test (SSRT) or a CERT? Where does Hydrogen like to go in a specimen (under stress) containing a crack? If SCC occurs by an active path mechanism, what happens if the sample is anodically polarised during the SCC test? What will happen if you cathodically polarise the SCC sample?

Forms of Corrosion Uniform or general corrosion Galvanic corrosion (2 dissimilar metals) Pitting corrosion (localised corrosion on flat surface) Crevice corrosion (geometry; occluded region) Intergranular corrosion (preferential corrosion) De-alloying (selective corrosion of an element) Erosion corrosion (wear + corrosion) Flow-Assisted Corrosion Microbial-Induced Corrosion (MIC) -- Environmentally-Assisted Cracking (not pure corrosion)

Corrosion: How to control it? Select a corrosion-resistant material Use a coating (paint) (barrier layer!) Use an inhibitor (added to solution or surface) Cathodic protection (external applied potential) Design to avoid corrosion