Corrosion can be defined as the deterioration of material by reaction to

its environment.

Corrosion occurs because of the natural tendency for most metals to

return to their natural state; e.g., iron in the presence of moist air will

revert to its natural state, iron oxide.

4 required components in an electrochemical corrosion cell:

1) An anode; 2) A cathode; 3) A conducting environment for ionic

movement (electrolyte); 4) An electrical connection between the anode and

cathode for the flow of electron current.

If any of the above components is missing or disabled, the

electrochemical corrosion process will be stopped.

This example illustrates some of the basics of corrosion.

On the surface of the Zn

bar we have the following

eZnZn 22

On the surface of the Cu bar

we have the following

CueCu 22

Note the current path. The salt bridge provides for ion exchange.

• Ranking the reactivity of metals/alloys in seawater

Platinum

Gold

Graphite

Titanium

Silver

316 Stainless Steel (passive)

Nickel (passive)

Copper

Nickel (active)

Tin

Lead

316 Stainless Steel (active)

Iron/Steel

Aluminum Alloys

Cadmium

Zinc

Magnesium

more

anodic

(act

ive)

more

cat

hodic

(iner

t)

Based on the appearance of the corroded metal, wet

corrosion may be classified as:

Uniform or General

Galvanic or Two-metal

Pitting

Environment-assisted cracking

Intergranular

Crevice

Velocity-assisted

Dealloying

Fretting

This one is common in steel that is unprotected by

any surface coating. Most noticeable. Surface effect,

leaving rust on the surface.

The good thing about this, if there is one, is that the

corrosion is widely spread around.

Corrosion over the entire

exposed surface at a

uniform rate. e.g..

Atmospheric corrosion.

Maximum metal loss by

this form.

Not dangerous, rate can

be measured in the

laboratory.

EXAMPLES:

1.rusting of iron 2.tarnishing

of silver 3.Fogging of nickel

4.high - temperature

oxidation of metals

Possibility when two dissimilar metals are electrically connected in an electrolyte

Results from a difference in oxidation potentials of metallic ions between two or more metals. The greater the difference in oxidation potential, the greater the galvanic corrosion.

Refer to Galvanic Series

The less noble metal will corrode (i.e. will act as the anode) and the more noble metal will not corrode (acts as cathode).

Perhaps the best known of all corrosion types is galvanic corrosion, which occurs at the contact point of two metals or alloys with different electrode potentials.

When two dissimilar metals are joined together and exposed, the more active of the two metals corrode faster and the nobler metal is protected. This excess corrosion is due to the galvanic current generated at the junction

Fig. Al sheets covering underground Cu cables

It is based on low oxygen concentration at the bottom of the pit.

This is very common in materials that protect themselves with a passive layer, i.e. stainless steel and aluminum.

Highly localized. Goes

deep into the metal.

Chloride ions find their way

into the pits, intensifies the

situation.

A form of extremely

localized attack causing

holes in the metal

Most destructive form

Autocatalytic nature

Difficult to detect and

measure

Mechanism

Pitting

Pitting is a localized form of corrosive

attack. Pitting corrosion is typified by the

formation of holes or pits on the metal

surface. Pitting can cause failure, yet the

total corrosion, as measured by weight loss,

may be minimal.

5th Century sword

Boiler tube

304 stainless steel /

acid chloride solution

Questions:

1. Worst combination?

2. Aluminum and steel?

3. Titanium and Zinc?

4. Stainless Steel and

Copper?

5. Mild steel and cast iron?

Galvanic Series:

GALVANIC SERIES

Platinum

Gold

Zirconium Graphite

Titanium

Hastelloy C Monel

Stainless Steel (316-passive)

Stainless Steel (304-passive)

Stainless Steel (400-passive)

Nickel (passive oxide)

Silver

Hastelloy 62Ni, 17Cr

Silver solder

Inconel 61Ni, 17Cr

Aluminum (passive AI203)

70/30 copper-nickel

90/10 copper-nickel

Bronze (copper/tin)

Copper

Brass (copper/zinc)

Alum Bronze Admiralty Brass

Nickel

Naval Brass Tin

Lead-tin

Lead

Hastelloy A

Stainless Steel (active)

316 404 430 410

Lead Tin Solder

Cast iron

Low-carbon steel (mild steel)

Manganese Uranium

Aluminum Alloys

Cadmium

Aluminum Zinc

Beryllium

Magnesium

Note, positions of

ss and al

Big Cathode, Small Anode = Big Trouble

When a metal is subjected to a tensile stress

and a corrosive medium, it may experience

Environment Assisted Cracking. Four types:

Stress Corrosion Cracking(SCC)

Hydrogen Embrittlement

Liquid Metal Embrittlement

Corrosion Fatigue

Static tensile stress and

specific environments

produce cracking

Examples:

1) Stainless steels in hot

chloride

2) Ti alloys in nitrogen

tetroxide

3) Brass in ammonia

Ingredients:

(1) tensile stress in the metal

(2) corrosive (electrolyte) environment.

Accelerators: presence of Chloride ion and high

temp.

Victims: Stainless steel is unsafe in water above

50C and over a few ppm of chloride, if any

tension exists. Others: mild steel in alkaline

environment, copper alloys in ammonia env.

The anode is the stresses region.

This is not exactly galvanic corrosion, but it definitely is a form of

environmental attack.

Hydrogen atoms diffuse into the metal from outside. Deep in the metal,

they combine to form H2 gas or combine with C, if present , to form CH4.

The pressure in this internal pockets of gas is enough to initiate cracking.

The metal is already seeing a lot of tensile stress.

Normally ductile high strength metals, particularly steels, are not so

ductile anymore because of these internal cracks.

High strength materials

stressed in presence of

hydrogen crack at

reduced stress levels.

Hydrogen may be

dissolved in the metal or

present as a gas outside.

Only ppm levels of H

needed

This is a segue from the previous. It is closely related.

Again, stainless steel is the ideal victim here. The

problem is triggered by improper heating, and often

this comes with welding. Carbides of chromium form

in the grain boundary regions.

The chromium is tied up in the carbides. It can’t

protect by forming the passive layer.

PLUS, there is a dissimilarity in metals producing a

small but definite galvanic corrosion.

Intensive localized

corrosion within

crevices & shielded

areas on metal

surfaces

Small volumes of

stagnant corrosive

caused by holes,

gaskets, surface

deposits, lap joints

Narrow and confined spaces.

Crevice Corrosion

This is a concentration cell in action. Notice how the damage

occurs in out of sight places.

Fast moving corrosives cause:

a) Erosion-Corrosion

b) Impingement attack

c) Cavitation damage in

metals

This is caused by the impingement of a high velocity turbulent flow on a surface.

The flow is often multi-phase. This means there can be entrained solid particles, or even gas bubbles, as in cavitation of a propeller.

The flow will carry away any protective layer that was intended to protect the material, and even abrade the flow surface.

Forms of Erosion:

› Liquid Impingement

› Liquid erosion

› Slurry Erosion

› Cavitation

Combined chemical attack and

mechanical wear

(e.g., pipe elbows)

Erosion-corrosion

Brass water pump

When one element in an alloy is anodic to the

other element.

Example: Removal of zinc from brass (called

dezincification) leaves spongy, weak brass.

Brass alloy of zinc and copper, and zinc is anodic

to copper (see galvanic series).

Cathodic protection: Make the structure more cathodic by

› Use of sacrificial anodes

› Impressed currents

› Galvanized steel

Anodic protection: Make passivating metal structures

more anodic by impressed potential. e.g. 316 s.s. pipe

in sulfuric acid plants.

Galvanization of Steel

Dip steel sheet in molten zinc. Get a pretty thin coating.

Zinc will be anode. Steel exposed by crack is the cathode. Since we have a huge anode having to be served by a small cathode, corrosion rate will be slow. Tiny cathode (steel)

Large area

anode (zinc)

An example of a favorable area ratio. Bad deal: huge cathode, tiny anode

Zinc is attached to the steel hull of the vessel.

Attachment points

Aluminium anodes mounted on a steel jacket

structure – using galvanic corrosion for

corrosion control! Called cathodic protection

(aka sacrificial anode)

By imposing a voltage which causes electrons to flow towards the

object to be protected, we make it less anodic and protect it from

corrosion damage.