CORROSIONCORROSION

SCHOLAR NAME - S.GNANASEKARAN

Under the guidance - Dr. G. PADMANABAN Dr. V. BALASUBRAMANIAN

UNIT IV: CORROSION BEHAVIOURDefinition and classification of Corrosion, Introduction to

corrosion principles, corrosion rate Expressions, Electro chemical Aspects: Electro chemical Reactions, polarization, and passivity. Environmental Effects: Effect of Oxygen and oxidizers, Velocity, Temperature, Corrosive Concentration, Galvanic Coupling & Metallurgical aspects (Metallic properties). Forms of Corrosion: Galvanic, Crevice, Pitting, Inter granular, Selective leaching, Erosion, Stress corrosion, Hydrogen damage (Definition and mechanisms only).

Definition

“The destruction of a material by chemical or electrochemical reaction to its environment”

Typically a transfer of electrons from one metal to another through an Oxidation-Reduction Reaction.

Losses due to Corrosion

Forms of Corrosion

1. Galvanic corrosion

2. Crevice corrosion

3. Pitting corrosion

4. Intergranular Corrosion

5. Erosion-corrosion

6. Hydrogen damage

GALVANIC CORROSIONGALVANIC CORROSION

1. Galvanic Galvanic or Dissimilar metal corrosion

Takes place when two metals are in physical contact with each other and are immersed in a conducting fluid. (electrically connected)

Examples:

1. Plate and screw (different electrical potentials).2. domestic water heater (Copper and steel )

Fundamental Requirements :

1. Dissimilar metals (or other conductors).

2. Electrical contact between the dissimilar conducting materials.

3. Electrolyte (the corrosive medium) in contact with the dissimilar conducting materials.

Galvanic corrosion between stainless steel screw and Aluminium.

Galvanic corrosion between Steel and Brass.

Anodic- cathodic behavior of steel with zinc and tin outside layers exposed to the Atmosphere (0.05M NaCl medium). (a) zinc is anodic to steel and corrodes (b) steel is anodic to tin and corrodes (the tin layer was perforated before the corrosion began)

Environmental Effects

Area Effect

(steel)

(Copper)

(steel)

(Copper)

Copper

(a)

(b)

Distance Effect

Accelerated corrosion due to galvanic effects is usually greater near the junction, with attack decreasing with increasing distance from that point.

The distance affected depends on the conductivity of the solution.

Eg. Current flow and the resistance of the circuits

Signs of Galvanic Corrosion

• Blistering of paint– 1st Warning Sign

• Formation of powdery substance– 2nd Warning Sign

• Pitting of metal– Too late– Severe Galvanic Corrosion

Prevention1. Select combination of metals as close together as possible in the galvanic

series.

2. Area of Metals Good – applying a less noble metal to a large area Bad – applying a more noble metal to a larger area– Steel screws / bolts on large bronze – Insulate dissimilar metals wherever possible.

3. Indirect Cathodic Protection Used when direct contact not possible

– Zinc bolted to outside of hull

4. Resistance of Electrical Path

Fresh water is less conductive than salt water

– Less galvanic current

– Use magnesium sacrificial anodes

CREVICE CORROSIONCREVICE CORROSION

2. Crevice CorrosionCrevice Corrosion

Intensive Localized corrosion usually associated with a stagnant solution on the micro-environmental level.

Also known as deposit or gasket corrosion.

Occurs in crevices (shielded areas) such as under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps.

Extremely dangerous because it is localized and can lead to component failure while the overall material loss is minimal.

The initiation and progress of crevice corrosion can be difficult to detect.

Stage 1The O2 content in the H2O occupying a crevice is equal to the level of soluble oxygen

Stage 2The corrosion reactions in the crevice (anodic) and on the open surface (cathodic).

Stage 31. The metal ions hydrolyze giving off protons (acid) and forming

corrosion products. 2. Serious increase in the corrosion rate 3. The accumulation of positive charge in the crevice becomes a

strong attractor to negative ions in the environment.

Mechanism

Environmental factors

Depending on the environment developed in the crevice and the nature of the metal, the crevice corrosion can take a form :

Pitting (i.e., formation of pits)

Filiform corrosion (this type of crevice corrosion that may occur on a metallic surface underneath an organic coating)

Intergranular attack

Stress corrosion cracking.

Steel components in open atmospheric environment. .

Marine environment

Underside of Air craft panel Close-up picture showing the

severity of corrosion

Prevention Use welded butt joints instead of riveted or bolted joints in new

equipment.

Avoid sharp corners and stagnant areas.

Provide uniform environment.

PITTING CORROSIONPITTING CORROSION

3 Pitting Pitting CorrosionCorrosion A localized form of corrosive attack that produces holes or small

pits in a metal.

Generally, the bulk surface remains unattacked.

Often found in situations where resistance against general corrosion is conferred by passive surface films.

Localized pitting attack is found where these passive films have broken down.

one of the most dangerous forms of corrosion because it is difficult to anticipate and prevent, relatively difficult to detect, occurs very rapidly, and penetrates a metal without causing it to lose a significant amount of weight.

Pitting is initiated byPitting is initiated by:

1. Localized chemical or mechanical damage to the protective oxide film; low dissolved oxygen concentrations (protective oxide film less stable) and high concentrations of chloride (as in seawater)

2. Localized damage or poor protective coating.

3. The presence of non-uniformities in the metal structure of the component, e.g. nonmetallic inclusions.

THROUGH PITS SIDEWAY PITS

Narrow, deep

Shallow, wide

Elliptical

Vertical Grain Attack

Subsurface

Undercutting

Horizontal grain attack

Corrosion Pit Shapes

MechanismLocalized corrosion developing on metal in a O2 environment

•It is an autocatalytic process.

•Metal oxidation results in localized acidity, maintained by the cathode and anode

•Which creates a potential gradient and electromigration of aggressive anions into the pit.•Polished surfaces display higher resistance to pitting.

A local cathode cell that leads to A local cathode cell that leads to the initiation of a pit the initiation of a pit

Prevention (Crevice and Pitting)

Prefer welding as a replacement of rivets & bolts

Avoid stagnant areas

Provide uniform environments

Use solid non-absorbent gaskets (Teflon)

INTERGRANULAR INTERGRANULAR CORROSIONCORROSION

4. 4. Intergranular CorrosionIntergranular Corrosion

Localized corrosion along grain boundaries or adjacent to grain boundaries, while the bulk of the grains remain largely unaffected.

Usually associated with chemical segregation effects or specific phases precipitated on the grain boundaries.

Sensitized stainless steels and aluminum alloys represent example of this form of damage.

Intergranular corrosion of a failed aircraft component

Severe problem in the welding of stainless steels (weld decay).

Knife line attack (KLA) Influences steels stabilized by niobium, such as 347 stainless steel. Titanium, niobium, and their carbides dissolve in steel at very high temperatures. At some cooling systems, niobium carbide does not precipitate and the steel then behaves like unstabilized steel, forming chromium carbide as an alternative.This affects only a thin zone several millimeters wide, making it difficult to spot and increasing the corrosion speed.

Sensitization effectPrecipitation of carbides at grain boundaries in a stainless steel or alloy subject to intergranular corrosion or intergranular stress corrosion cracking.In a corrosive atmosphere, the grain interfaces of these sensitized alloys become very reactive to intergranular corrosion. This is characterized by a localized attack at and adjacent to grain boundaries with relatively little corrosion of the grains themselves.

Sensitized microstructure

Normal microstructure

EROSION CORROSION

5. EROSION-CORROSION (“Flow-Assisted” or “Flow-Accelerated” Corrosion)

An increase in corrosion carried by a high relative velocity between the corrosive environment and the surface.

Removal of the metal:– As corrosion product which “spalls off” the surface because of the

high fluid shear and bares the metal beneath;– As metal ions, which are swept away by the fluid flow before they

can deposit as corrosion product.

Difference between erosion-corrosion and erosion:– Erosion is the straight forward wearing away by the mechanical

cut caused by suspended particles . . . e.g., sand-blasting, erosion of turbine blades by droplets .

– Erosion-corrosion also involves a corrosive environment . The metal undergoes a chemical reaction.

Erosion-corrosion produces a typical surface finish

grooves, waves, holes, etc., all oriented with respect to the fluid flow

Erosion-corrosion of condenser tube wall.

Erosion-corrosion of stainless alloy pump impeller.

Attack occurs when film cannot form because of erosion caused by suspended particles, or when rate of film formation is less than rate of dissolution and transfer to bulk fluid.

Mechanism

Erosion-Corrosion found in: - water contaning solutions;

- gases;- organic liquids;- liquid metal.

If fluid contains suspended solids, erosion-corrosion may be aggravated.

Weak equipment :

- pipes (bends, elbows, tees);- valves;- pumps;- blowers;- propellers, impellers;- stirrers;- stirred vessels;- HX tubing (heaters, condensers);

- flow-measuring orifices, venturies;- turbine blades;- nozzles;- baffles;- metal-working equipment (scrapers, cutters, grinders, mills);- spray impingement components;- etc.

Prevention of Erosion-Corrosion

Design (avoid impingement, high velocity, etc.)

Materials (use Cr-containing steels)

Use hard corrosion-resistant coatings.

HYDROGEN DAMAGE

6. Hydrogen damage

Generic name given to a large number of metal destroyed

due to interaction with hydrogen.

Hydrogen Effects

Hydrogen can degrade metals by:

• Hydrogen blistering

• Hydrogen embrittlement

• Decarburization

• Hydrogen attack.

destroyed

BlisteringHydrogen enters the lattice of a metal, diffuses to voids, creates high internal stresses blisters . . .

Blistering may occur during exposure to: • Hydrocarbons• Electroplating solutions• Chemical process streams• Pickling solutions• H-containing contaminants during welding• General corrosive environments.

Carbon steel plate showing a large hydrogen blister. Exposure time: 2 years.

Mechanism of hydrogen blistering

The mechanism of hydrogen uptake by metals must

involve atomic hydrogen.

Molecular hydrogen cannot diffuse through metal lattices.

Mechanism of hydrogen blistering.

Prevention of Blistering

Use coatings

Remove impurities that can promote hydrogen

evolution .

Use different materials (Ni-base alloys have low

diffusion rates for hydrogen).

EmbrittlementSimilar to blistering . . . hydrogen enters metal lattice. High-strength (and more brittle) steels are susceptible.

H-embrittlement different from SCC in nature of cracks . . . stress-corrosion cracks usually propagate anodically.

Prevention of Embrittlement

Reduce corrosion rate (surface treatment, coatings, etc.)

Change electroplating process to minimize H effects

(voltage, current density, bath composition, etc.)

Heating the material to remove H

Minimize residual stresses

Use less susceptible material

Maintain clean conditions during welding.

Hydride-forming metals

Are susceptible to H- embrittlement . . .e.g., Zr-alloy pressure tubes.

These hydrides are themselves brittle, and crack.

The crack can propagate through the material, with more hydride

progressively precipitating at the crack tip.

Pressure tube-Crack propagation

Hydrogen Attack

High temperature process - C or carbide in steels can react with gaseous hydrogen . . .

C + 2H2 CH4

May crack the steel from high internal pressure.

May cause loss of strength as C disappears.

Thanking you