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MIAT 105 Materials Metals Day 5

Fifth Lecture on Corrosion
by

MIAT 105 WandM

on 30 July 2013

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Transcript of MIAT 105 Materials Metals Day 5

Materials Metals Day 5
In the most common use of the word, CORROSION means electrochemical oxidation of metals in reaction with an oxidant such as oxygen.

This type of damage typically produces oxide(s) and/or salt(s) of the original metal.

A very common example of electrochemical corrosion is the rusting which is the formation of an oxide of iron due to oxidation of the iron atoms.

Corrosion can also refer to other materials than metals, such as ceramics or polymers, although in this context, the term degradation is more common.

In other words, corrosion is the wearing away of metals due to a chemical reaction.
Corrosion (cont.)
Galvanic corrosion is affected by:
anode/cathode area ratio
types of metal
operating conditions (temperature, humidity, salinity, etc.)

The surface area ratio of the anode and cathode will directly affect the corrosion rates of the materials.

A small anode/cathode area ratio is highly undesirable.
In this case, the galvanic current is concentrated onto a small anodic area.
Rapid thickness loss of the dissolving anode tends to occur under these conditions.
Galvanic Corrosion (cont.)
This type of corrosion occurs under painted or plated surfaces when moisture permeates the coating.
- Lacquers and "quick-dry" paints are most susceptible to the problem.

Filiform corrosion normally starts at small, sometimes microscopic, defects in the coating.

It appears as a radial "worm-like“ corrosion path emanating from a central core of corrosion.
Filiform Corrosion
Pitting is one of the most destructive and insidious forms of corrosion.

It causes equipment to fail because of perforation with only a small percent weight loss of the entire structure.

Pitting remains among the most common and damaging forms of corrosion in passivated alloys (such as stainless steel).

It can be prevented by control of the alloy's environment, which often includes ensuring that the material is exposed to oxygen uniformly (i.e. eliminating crevices).
Pitting Corrosion (cont.)
Pitting is initiated by:

Localized damages to the protective oxide film
mechanical damage
chemical damage
water chemistry factors which can cause breakdown of a passive film are: acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater)

Localized damage to, or poor application of, a protective coating

The presence of non-uniformities in the metal structure of the component, e.g. nonmetallic inclusions
Pitting Corrosion (cont.)
The different forms of corrosion represent corrosion phenomena categorized according to their appearance:

Group 1 - readily identifiable by ordinary visual examination
Uniform corrosion
Pitting
Crevice corrosion
Filifom corrosion
Galvanic corrosion

Group 2 - may require supplementary means of examination
Intergranular corrosion
Exfoliation
Dealloying (selective leaching)
Fretting corrosion
Erosion corrosion
Cavitation

Group 3 - verification is usually required by microscopy (optical, electron microscopy etc.)
Environmental cracking
Stress Corrosion Cracking
Corrosion fatigue
Hydrogen embrittlement
Forms of corrosion
Some metals form passive films on their surfaces and these prevent, or slow down, their corrosion process.
Titanium Oxide
Stainless Steel

We can prevent corrosion by using metals that form naturally protective passive films, but these alloys are usually expensive or may not provide all the mechanical characteristics required for a specific application.
Corrosion (cont.)
Corrosion can be defined as the degradation of a material due to a reaction with its surrounding environment.


Degradation implies deterioration of physical properties of the material.


Materials can be metals, polymers (plastics, rubbers, etc.), ceramics (concrete, brick, etc.) or composites (mechanical mixtures of two or more materials with different properties, e.g. GRP).
Corrosion
Corrosion
Galvanic corrosion also called ‘dissimilar metal corrosion’ or ‘two-metal corrosion’ refers to corrosion damage induced when two dissimilar (different) materials are coupled in a corrosive electrolyte .

For galvanic corrosion to occur, 3 conditions must be present:
- Electrochemically dissimilar metals must be present

- These metals must be in electrical contact (may be made by metal itself or by metallic connection such as a bolt or a rivet)

- The metals must be exposed to an electrolyte
Galvanic Corrosion
Pitting Corrosion (cont.)
Intergranular corrosion is a localized attack along the grain boundaries, or immediately adjacent to grain boundaries of a metal or alloy, while the bulk of the grains remain largely unaffected.
Intergranular Corrosion
Control of galvanic corrosion is achieved by:

using metals closer to each other in the galvanic series.

- electrically isolating metals from each other, using nonconductive barrier materials, a paint coating, or by plating.
Galvanic Corrosion (cont.)
This arrangement of metals determines what metal will be the anode and cathode when the two are put in a electrolytic cell (arrangement dependent on salt water as electrolyte).
The relative nobility of a material can be predicted by measuring its corrosion potential in the medium of interest (i.e. electrolyte).


This hierarchy is called a galvanic series, and most commonly lists the relative nobility of certain materials in sea water.


Relative nobility shows how active a metal is in a certain medium in releasing its atoms in form of metallic ions to enter into solution (electrolyte).
Galvanic Corrosion (cont.)
In a given environment (i.e. electrolyte), one metal will be either more noble (cathodic) than the next, or less noble (more active or anodic) than the next, based on how strongly its ions are bound to the surface.

The noble metal will tend to take electrons from the active one, while the electrolyte hosts a flow of ions in the same direction.
When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would by itself, while the other becomes the cathode and corrodes slower than it would alone.
Galvanic Corrosion (cont.)
Electrolyte = any solution that conducts electric current and contains negative an positive ions, such as fresh or salt water and acid or alkaline solutions of any concentration.
When aluminum alloys or magnesium alloys are in contact with steel (carbon steel or stainless steel), galvanic corrosion can occur and accelerate the corrosion of the aluminum or magnesium.
Galvanic Corrosion (cont.)
Filiform corrosion is minimized by careful surface preparation prior to coating and by the use of coatings that are resistant to this form of corrosion.
Filiform corrosion shows itself as a puffiness under the paint film.
When the paint film is broken it may be noticed that the puffiness was caused by the growth of the powdery oxide or salt of corrosion.
Filiform Corrosion (cont.)
Screws and fasteners are common sources of crevice corrosion problems.

Cleanliness, the proper use of sealants, and protective coatings are effective means of controlling this problem.
Crevice Corrosion (cont.)
Crevice corrosion is initiated by changes in local chemistry within the crevice:

- Depletion of inhibitor in the crevice

- Depletion of oxygen in the crevice

- A shift to acid conditions in the crevice

- Build-up of aggressive ion species (e.g. chloride) in the crevice
Crevice Corrosion (cont.)
If allowed to continue, the surface becomes rough and possibly frosted in appearance.
Uniform corrosion (or general corrosion) is characterized by corrosive attack proceeding evenly over the entire surface area, or a large fraction of the total area.

On a polished surface, this type of corrosion is first seen as a general dulling of the surface.
Uniform Corrosion
All other metals, to include iron-the metal most commonly used, or aluminum, are processed from minerals or ores into metals which are inherently unstable in their environments.

They are unstable and have a tendency to revert to their more stable mineral forms.
Metals corrode because we use them in environments where they are chemically unstable.

Only copper and the precious metals (gold, silver, platinum, etc.) are found in nature in their metallic state.
Corrosion (cont.)
Intergranular corrosion of a failed aircraft component made of 7075-T6 aluminum (picture width = 500 mm)
The attack usually progresses along a narrow path along the grain boundary and, in a severe case of grain-boundary corrosion, entire grains may be dislodged due to complete deterioration of their boundaries.
The attack can be caused by impurities at the grain boundaries, enrichment of one of the alloying elements, or depletion of one of these elements in the grain-boundary areas.

In certain environmental conditions these elements make the grain boundary zone anodic relative to the remainder of the surface initiating so the corrosion.
Intergranular Corrosion (cont.)
Galvanic corrosion can be recognized by the presence of a buildup of corrosion at the joint between the dissimilar metals.
Galvanic Corrosion (cont.)
Stagnant microenvironments tend to occur in crevices (shielded areas) formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps.
Crevice or contact corrosion is the corrosion produced at the region of contact of metals with metals or metals with nonmetals.

Usually crevice corrosion is associated with a stagnant solution on the micro-environmental level.
Crevice Corrosion
Pitting is a form of extremely localized attack that results in holes in the metal.

These holes may be small or large in diameter, but in most cases they are relatively small.

Generally a pit may be described as a cavity or hole with the surface diameter about the same as or less than the depth.
Pitting Corrosion
Surface corrosion can lead to more serious types of corrosion.
As corrosion occurs uniformly over the entire surface of the metal component, it can be practically controlled by cathodic protection, use of coatings or paints, or simply by specifying a corrosion allowance.
Uniform corrosion is relatively easily measured and predicted, making disastrous failures relatively rare.

In many cases, it is objectionable only from an appearance standpoint.
Uniform Corrosion (cont.)
- Copper or copper alloy (brass, bronze) surfaces will tarnish to a gray-green film called as patina which is smooth and offers protection.
Verdigris
- Copper  and  its  alloys  are  generally  corrosion resistant,   although   the   products   of   corrosive   attack   on copper  are  commonly  known.

- For example the exposure of copper or copper alloys to salt spray will cause the formation of blue or green salts called verdigris, which is indicative of active corrosion.
Corrosion (cont.)
Most often it is seen on extruded sections where grain thickness is less than in rolled forms.

The damage often initiates at end grains encountered in machined edges, holes or grooves and can subsequently progress through an entire section.
Exfoliation is a form of intergranular corrosion that progresses parallel to the metal surface in such a manner that underlying layers are gradually separated.
Exfoliation Corrosion
This type of corrosion is most commonly the result of welding.
Some types of stainless steels sensitized by welding or brazing may develop intergranular corrosion.

Also, some aluminum alloys especially which have been extruded or otherwise worked heavily, with a microstructure of elongated and flattened grains are particularly prone to this damage.
In any case the mechanical properties of the structure will be seriously affected.
Intergranular Corrosion (cont.)
Corrosion on steel beams
Corrosion on bolts
Corrosion on pipes
Corrosion (cont.)
Cavitation is considered a special case of erosion corrosion.

Cavitation is caused by the formation and collapse of vapor bubbles in a flowing liquid close to a solid (e.g. metal) surface.

Vapor bubbles emerge in the regions where the pressure of the liquid falls below its vapor pressure, and they contain air and or other gases that are present (dissolved) into the liquid.

Usually, in a flowing liquid low pressure regions are associated with high fluid speeds.

The formation of the vapor bubbles in cavitation is caused by the pressure decrease unlike that in boiling when bubbles emerge as a consequence of temperature increase.
Cavitation
Erosion corrosion can be controlled by :
the use of harder alloys or a more corrosion resistant alloy

the alterations in fluid velocity and changes in flow patterns

filtering out solid particles

controlling the oxygen content into the fluid

lowering the fluid temperature
Erosion Corrosion (cont.)
In two-phase liquids (containing suspended solid particles or gas bubbles), the impact of the particles can damage or even eliminate the protective layers or passive films that are normally stable in the absence of particles, and the local corrosion rate is then markedly accelerated.

Erosion corrosion is characterized in appearance by grooves, gullies, waves, rounded holes, and valleys and usually exhibits a directional pattern (comet tails, horseshoe marks, etc) .

Surfaces which have undergone erosion corrosion are generally fairly clean, unlike the surfaces from many other forms of corrosion.

Erosion corrosion is the second most common cause of copper tube failure.
Erosion Corrosion (cont.)
Most metals and alloys can be affected, particularly soft materials (e.g. copper, lead, etc.) or those whose corrosion resistance depends on the existence of a surface film (aluminum, stainless steels).

Erosion corrosion is often the result of the wearing away of a protective scale or coating on the metal surface.

The fluid turbulence and flow rate influence the erosion rates.

The increased turbulence caused by pitting on the internal surfaces of a tube can result in rapidly increasing erosion rates.
Erosion Corrosion (cont.)
It can occur in structural members such as trusses where highly loaded bolts are used and some relative motion occurs between the bolted members.

Fretting corrosion is greatly retarded when the contacting surfaces can be well lubricated as in machinery-bearing surfaces so as to exclude direct contact with air.
Fretting Corrosion (cont.)
Environmental cracking refers to a corrosion cracking caused by a combination of conditions (mechanical + chemical) that can specifically result in one of the following form of corrosion damage:

- Stress Corrosion Cracking (SCC)

- Corrosion fatigue

- Hydrogen embrittlement
Environmental Cracking
The vapor bubbles are transported by the fluid flow and when a low pressure region is reached they implode/collapse (inward explosion) generating shock wave and micro-jets that will hit the solid surface eroding it.

Calculations have shown that the implosions produce shock waves with pressures approaching 415MPa (ca.60,000psi)
Cavitation (cont.)
Erosion corrosion is the result of a combination of an aggressive chemical environment and high fluid-surface velocities.

This can be the result of fast fluid flow past a stationary object (e.g. a check valve, a pipe elbow, etc.) or it can result from the quick motion of an object in a stationary fluid (ship's propeller) .
Erosion Corrosion
Fretting corrosion is the rapid corrosion that occurs at the interface between contacting, highly loaded metal surfaces when subjected to slight vibratory motions.

This type of corrosion is most common in bearing surfaces in machinery, such as connecting rods, splined shafts, and bearing supports, and often causes a fatigue failure.

Pits or grooves and oxide debris characterize this damage.
Fretting Corrosion
Dealloying or selective leaching is a rare form of corrosion found in copper alloys, gray cast iron, and some other alloys.

Dealloying 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.

The selective removal of the active component of the metal can proceed in a uniform manner (layer type) or on a localized (plug type) scale.
Dealloying
(selective leaching)
Sometimes it known as lamellar or layer corrosion.

In extreme cases, the edges of the affected area are leaf like and resemble the separated pages of a wetted book that has become swollen and begun to open up.
Exfoliation Corrosion (cont.)
Cavitation is undesirable because it can produces extensive erosion by removing protective surface scales.
Severe cavitation is always accompanied by noise from the resultant knocking, vibrations, and a significant reduction of the efficiency because it distorts the flow pattern.

Cavitation my develop in pipelines, in hydraulic machines (turbines, pumps, and propellers), hydraulic circuit components (e.g. valves), dam spillways, diesel engines, submerged hydrofoils, etc.
Cavitation (cont.)
Erosion Corrosion (cont.)
Fretting corrosion caused by marginal lubrication on chain pins
Extensive fretting corrosion on the outer race of a deep groove ball bearing
Fretting Corrosion (cont.)
Graphitized gray cast iron pipe showing graphitic plugs on its cut surface
Another example of selective leaching is the graphitic corrosion of gray cast iron.

In this case the selective removal of iron will lead to a porous graphite network impregnated with insoluble corrosion products.

As a result, the cast iron retains its appearance and shape but is weaker structurally.
Dealloying (cont.)
Virtually all copper alloys are subject to dealloying in some environments.
In-service valve suffering from dezincification has a white powdery substance or mineral stains on its exterior surface
Dezincified brass leaving a porous copper plug on the surface
A common example of dealloying corrosion is the dezincification of unstabilized brass, whereby a weakened, porous copper structure is produced.
Dealloying (cont.)
Fretting corrosion of a fence post and wires which swing in the wind and wear against the post
Ball bearing outer ring with fretting corrosion and longitudinal crack in a deep groove
Spherical roller bearing inner ring with fretting corrosion and a transverse crack right through the ring. The fretting corrosion has caused the cracking.
Fretting Corrosion (cont.)
Cavitation on an automotive water pump case
Cavitation corrosion of a deaerator
Cylinder Wall Cavitation Erosion
Cavitation of a nickel alloy pump impeller blade exposed to a hydrochloric acid medium.
Cavitation (cont.)
Cavitation damages on a valve plate of an axial piston hydraulic pump
Cavitation damage to a axial flow pump
Cavitation damage to a Francis turbine
Cavitation damage often appears as a collection of closely spaced, sharp-edged pits or craters on the surface.
Cavitation (cont.)
Corrosion fatigue is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion.

No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment.

Damage from corrosion fatigue is greater than the sum of the damage from both cyclic stresses and corrosion.
Corrosion Fatigue
Corrosion Fatigue (cont.)
Fatigue corrosion failure occurs in two stages.

During the first stage, the combined action of corrosion and cyclic stresses damages the metal by pitting and crack formation.
Corrosion Fatigue (cont.)
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.
Stress Corrosion Cracking
An infamous example of corrosion fatigue occurred in 1988 on an airliner flying between the Hawaiian islands.
This disaster, which cost one life, prompted the airlines to look at their airplanes and inspect for corrosion fatigue.
Corrosion Fatigue (cont.)
The "beach marks" on the propeller blade mark the progression of fatigue on this surface.
The second stage is essentially a fatigue stage in which failure proceeds by propagation of the crack and is controlled primarily by stress concentration effects and the physical properties of the metal.

The fracture by cyclic stressing will ultimately occur, even if the corrosive environment is completely removed.

Fracture of a metal part due to fatigue corrosion generally occurs at a stress far below the fatigue limit in laboratory air, even though the amount of corrosion is extremely small.

For this reason, protection of all parts subject to alternating stress is particularly important wherever practical, even in environments that are only mildly corrosive.
Corrosion Fatigue (cont.)
Hydrogen embrittlement seriously reduce the ductility and load-bearing capacity, cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials.
Hydrogen embrittlement does not affect all metallic materials equally.
The most vulnerable are high-strength steels, titanium alloys and aluminum alloys.
The embrittlement of a metal or alloy by atomic hydrogen involves the ingress of hydrogen into a component.
Hydrogen Embrittlement
SCC of an insulated stainless-steel condensate line
The cracks form and propagate approximately at right angles to the direction of the tensile stresses at stress levels much lower than those required to fracture the material in the absence of the corrosive environment.

As cracking penetrates further into the material, it eventually reduces the supporting cross section of the material to the point of structural failure from overload.
Stress Corrosion Cracking (cont.)
River branching pattern is unique to SCC
Intergranular SCC with the crack following the grain boundaries
Usually SSC nucleate at pits, are highly branched and has a transgranular crack pattern.
Stress Corrosion Cracking (cont.)
Microbial corrosion (bacterial corrosion), is a corrosion caused or promoted by microorganisms, usually chemoautotrophs.

It can apply to both metals and non-metallic materials, in both the presence and lack of oxygen.

Sulfate-reducing bacteria are common in lack of oxygen; they produce hydrogen sulfide, causing sulfide stress cracking.

In presence of oxygen, some bacteria directly oxidize iron to iron oxides (rust) and hydroxides.
Other bacteria oxidize sulfur and produce sulfuric acid causing sulfide corrosion.

Concentration cells can form in the deposits of corrosion products, causing and enhancing galvanic corrosion.
Microbial corrosion
Hydrogen Embrittlement of Stainless Steel
- Hydrogen diffuses along the grain boundaries and combines with the carbon, which is alloyed with the iron, to form methane gas.
- The methane gas is not mobile and collects in small voids along the grain boundaries where it builds up enormous pressures that initiate cracks.
- If the metal is under a high tensile stress, brittle failure can occur.
Hydrogen entry can be facilitated in a number of ways:

by some manufacturing operations such as welding, electroplating, phosphating and pickling

as a by-product of a corrosion reaction

the use of cathodic protection for corrosion protection if the process is not properly controlled.
Hydrogen Embrittlement (cont.)
Steel tank microbial corrosion damage (similar to pitting corrosion)
Titanic's bow exhibiting microbial corrosion damage
In addition to the use of corrosion resistant alloys, control of microbial corrosion involves the use of biocides and cleaning methods that remove deposits from metal surfaces.
Microbial corrosion (cont.)
This is a non-galvanic form of corrosion which can occur when a metal is subject to a high temperature atmosphere containing oxygen, sulfur or other compounds capable of oxidising (or assisting the oxidation of) the material concerned.

Some componets used in aerospace, power generation and even in car engines are exposed at high operating temperature and to an atmosphere containing potentially highly corrosive products of combustion.

The products of high temperature corrosion can potentially be turned to an advantage.

For example, the formation of oxides on stainless steels, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperature in hostile conditions.

Such high temperature corrosion products in the form of compacted oxide layer glazes have also been shown to prevent or reduce wear during high temperature sliding contact of metallic (or metallic and ceramic) surfaces.
High temperature corrosion
There are four basic methods for Corrosion Control & Corrosion Protection:

Barrier Protection
Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate.

Cathodic Protection
Employs protecting one metal by connecting it to another metal that is more anodic, according to the galvanic series.

Corrosion Resistant Materials
Materials inherently resistant to corrosion in certain environments.

Corrosion Inhibitors
Modify the operating environment.
Methods of Corrosion Control
Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.

It is a method used to control corrosion of surfaces that are immersed in water or exposed to soil (steel water, and fuel pipelines and tanks; steel pier piles, ships, offshore oil platforms, offshore wind turbine foundations, etc.)

There are 2 methods:
cathodic protection with galvanic sacrificial anodes

impressed current cathodic protection
Cathodic Protection
Steel articles are immersed in a bath of molten zinc (ca. 8300F)

> 98% pure zinc, minor elements added for coating properties (Al, Bi, Ni)

Zinc reacts with iron in the steel to form galvanized coating.
Galvanizing
Coatings are the most common anti-corrosion treatments.
They are classified as follows:
Organic and inorganic coating (i.e. paintings, laquers, etc)
Anodic coatings (anodizing)
Chatodic coatings (galvanic zinc, chrome, copper, nickel applications)
Inhibitive coatings (red lead, zinc chromate)
Barrier Protection
8-shaped descender (climbing equipment) annodized with a yellow finish
Anodizing is a process of thickening and toughening the metal’s oxide film.

On metals such as aluminum the ticker layer of oxides increases resistance to further corrosion.

If this coating is scratched, normal passivation processes take over to protect the damaged area.

Anodized films not only provides a hard wear surface but also a somewhat porous base for paint or other coatings.
Anodizing
-
+
Rectifier
External source of direct current power (Rectifier) is connected (or impressed) between the structure to be protected (cathode) and the ground bed/water anode.
Cathodic Protection - Impressed Current
-
+
Pieces of an active metal such as magnesium or zinc are placed in contact with the corrosive environment and are electrically connected to the structure to be protected
Cathodic Protection Galvanic Sacrificial Anode
Zinc Metallizing (plating)
Feeding zinc into a heated gun, where it is melted and sprayed on a structure or part using combustion gases and/or auxiliary compressed air

Zinc-rich Paints
Zinc-rich paints contain various amounts of metallic zinc dust and are applied by brush or spray to properly prepared steel

Hot-dip Galvanizing
Complete immersion of steel into a kettle/vessel of molten zinc
Cathodic Coating
Galvanic Zinc Application
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