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© Robert Gordon University and Owen S. Jenkins Ltd. 2010
Wells Module ENM201
3. Types of Corrosion
Prepared and Presented byOwen Jenkins.
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© Robert Gordon University© Robert Gordon University and Owen S. Jenkins Ltd. 2010
Topic Overview
● In this lecture we shall be considering the different types of corrosion, their morphology (i.e. what they look like) and we shall start to consider how they occurred.
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Types of Corrosion● Fontana identifies eight forms of corrosion (see
Chapter 3 of the vermilion volume):
1.Uniform, or general attack2.Galvanic, or two-metal corrosion3.Crevice corrosion4.Pitting5.Intergranular corrosion6.Selective leaching, de-alloying or parting (including
graphitisation)7.Erosion corrosion, and8.Stress corrosion.
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More Types of Corrosion
● We could also add loads more, but we’ll save them for the Materials and Corrosion Module, ENM233
● The first eight cover most that you will need to know about
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1. Uniform Corrosion
● Uniform, or general attack
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Photo above: © KSC Corrosion Lab. Used with permission.
Left: OSJL collection.
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2. Galvanic Corrosion
●Galvanic, or Two-Metal Corrosion■Stainless steel screw causing galvanic
corrosion of aluminium
Photo © KSC Corrosion Lab. Used with permission
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3. Crevice Corrosion● Crevice or contact corrosion is the corrosion
produced at the region of contact of metals with metals or metals with non-metals.■ Crevice corrosion of Ti flange (below)
Photo © KSC Corrosion Lab. Used with permission
© KSC Corrosion Lab. Used with permission.
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4. Pitting
Photos: Above and right: OSJL collection.Top right © KSC Corrosion Lab. Used with permission.
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5. Intergranular Corrosion
● Stainless steel corroded in the Heat Affected Zone a short distance from the weld. (Weld decay)
● Heat sensitization● Typical of
intergranular corrosion in austenitic stainless steels.
© KSC Corrosion Lab. Used with permission.
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5a. Exfoliation Corrosion.
● Exfoliation is a form of intergranular corrosion.
© KSC Corrosion Lab. Used with permission.
© KSC Corrosion Lab. Used with permission.10
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6. Selective Leaching● Selective leaching, De-alloying or Parting● A fairly rare form of corrosion found in copper alloys (e.g.
dezincification of brass) grey cast iron, and some other alloys.■ De-alloying occurs
when the alloy loses the active component of the metal and retains the more corrosion resistant component in a porous "sponge" on the metal surface.
■ It can also occur by redeposition of the noble component of the alloy on the metal surface. © KSC Corrosion Lab. Used with permission.
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6a. Graphitisation●Graphitisation
■ A form of leaching: the graphitisation of cast irons.
■ In slightly acidic waters both flake graphite (grey) and nodular graphite (ductile) irons are corroded due to the anodic behaviour of the matrix with respect to the cathodic graphite.
■ Ductile iron pipes were once assumed to be immune to graphitisation, but examples from both Ottawa and Toronto clearly show graphitisation in this pipe material.
■ The picture right shows an example of graphitisation in a ductile iron pipe.
Photo: Institute for Research in Construction - National Research Council Canada
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6b. De-Nickelification● Cupronickel tubes get their
corrosion resistance by the formation of a passive oxide film on the exposed surface.
● The stability of this passive film requires that oxygenated water maintain access to the surface.
● Heavy deposits on the bottom of this tube resulted in stagnant, non-oxygenated water in contact with the cupronickel.
● Once the de-nickelification is started, it can be self-propagating because access of oxygen to the pit environment is limited.
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Photos Copyright © Corrosion Testing Laboratories, Inc
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7. Erosion Corrosion
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● Mechanical wear effects or abrasion are usually involved as well.
● Erosion corrosion is the acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface.
Photos: OSJL collection.
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7. Erosion Corrosion
Copyright © Corrosion Testing Laboratories, Inc
●Photograph of erosion-corrosion showing individual teardrop shaped pits with undercutting in the downstream direction.
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7. Erosion Corrosion
● Schematic of turbulent eddy mechanism for downstream undercutting of erosion-corrosion pits.
Illustrations Copyright © Corrosion Testing Laboratories, Inc
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8. Stress Corrosion
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● Stress corrosion cracking (SCC) is caused by the simultaneous effects of tensile stress and a specific corrosive environment.
● Stresses may be due to applied loads, residual stresses from the manufacturing process, or a combination of both.
n Cross sections of SCC frequently show branched cracks.
n This ‘river branching’ pattern is unique to SCC and is used in failure analysis to identify when this form of corrosion has occurred.
Photo © KSC Corrosion Lab. Used with permission.
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8. Stress Corrosion● The microscopic intergranular
SCC of an aluminium aerospace part
● The intergranular nature of the corrosion can be seen in the scanning electron microscope image (lower)and in the cross section (upper)
● The arrows indicate the primary crack shown in both pictures.
● Note that secondary cracks are also apparent.
● Secondary cracks are common in stress corrosion cracking.
Photos © KSC Corrosion Lab. Used with permission.
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8. Stress Corrosion
● Section through the neck of a non-magnetic drill collar pin connection, which has been severed from the tool body for investigation.
● Red arrow points to a hairline stress-corrosion crack.
● Drill collars normally are subjected to high compressive and bending-under-compression loads, but relatively low tensile loads, except in the pin (male threaded end) connection. Photo: Owen S. Jenkins Ltd. Collection
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© Robert Gordon University© Robert Gordon University and Owen S. Jenkins Ltd. 2010
8 Types of Corrosion
1.Uniform, or general attack2.Galvanic, or two-metal corrosion3.Crevice corrosion4.Pitting5. Intergranular corrosion6.Selective leaching, de-alloying or parting
(including graphitisation)7.Erosion corrosion, and8.Stress corrosion.
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© Robert Gordon University© Robert Gordon University and Owen S. Jenkins Ltd. 2010
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Wells Module ENM201
3. Types of Corrosion