3 Types of Corrosion

© Robert Gordon University and Owen S. Jenkins Ltd. 2010

Wells Module ENM201

3. Types of Corrosion

Prepared and Presented byOwen Jenkins.

© 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.


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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© Robert Gordon University© Robert Gordon University and Owen S. Jenkins Ltd. 2010 8

4. Pitting

Photos: Above and right: OSJL collection.Top right © KSC Corrosion Lab. Used with permission.


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.


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5a. Exfoliation Corrosion.

● Exfoliation is a form of intergranular corrosion.


© KSC Corrosion Lab. Used with permission.10


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


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.



Copyright © Corrosion Testing Laboratories, Inc

●Photograph of erosion-corrosion showing individual teardrop shaped pits with undercutting in the downstream direction.

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● 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.

© Robert Gordon University© Robert Gordon University and Owen S. Jenkins Ltd. 2010 18

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.


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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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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Prepared by

OWEN S. JENKINS LTD.4, Charlton Avenue, Aboyne, Aberdeenshire, AB34 5GL, United

Kingdom. Tel. +44 (0)13398 87779. Mobile phone: +44 (0)7803 296779.

Email: 21

[email protected] Website: www.osjl.co.uk

Wells Module ENM201

3. Types of Corrosion

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3 Types of Corrosion

Documents

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