Corrosion 3

To change corrosion rates

reduce/accelerate either the anodic or cathodic reaction

Environmental effects can change reaction

Oxygen/Oxidizers

For most metals that do not form an unreactive oxide layer (pink)

• Adding INCREASES the corrosion rate

For metals that do not form an unreactive oxide layer, there is a point where additional oxygen/oxidizer has no effect (blue)

For metals originally exist with the unreactive oxide film, shift to active can occur IF very powerful oxidizers are added (purple)

For most metals that do not form an unreactive oxide layer

Adding oxygen or oxidizers INCREASES the corrosion rate

For metals that do not form an unreactive oxide layer, there is a point where additional oxygen/oxidizer has no effect (blue)

This occurs when the oxide layer has formed and corrosive attack can no longer occur (get through the oxide)

If the process controlled by reaction series

then velocity has no effect

For reaction series to control, species must first attach, then electron transfer, then release

If metal readily forms oxide film

then slow velocities keep corrosive ions near the surface, preventing film formation, while fast velocities form more oxides (pink)

Some metals form large bulk films

 but only weakly attached.

Prior to velocity breaking/peeling film, corrosion barely occurs. Once velocity has broken the adhered film, corrosion increases (red)

Increasing temperature

Increases corrosion rates

Some metals see a gradual increase (Blue)

Some metal see a minimal change, followed by a rapid increase

Concentration of corrosive Ions

Metals with an unreactive oxide film can withstand higher concentrations before the ions overwhelm the film (pink)

Film may also become soluble in solution at high concentrations

Concentration of Corrosive ions

Most common with acids; As acid concentration increases, hydrogen ions increase. At a certain point, ionization of hydrogen is reduced, reducing the amount of hydrogen available for reaction thus reducing corrosion create (blue)

 

H+ automatically becomes H2

Uniform Attack

Metals is evenly attacked across the entire surface

Metal thins until piece fails

Reaction sites move around surface, creating uniform disintegration

Greatest destruction of metal on a tonnage basis

Not a great technical issue

Lifespan determined by comparison to tests

Can be reduced by coatings, corrosion inhibitors, or cathodic protection

Localized Corrosion- Uniform Attack

Anodes and cathodes become fixed

Leads to formation of geometrical constrained concentrations points, which prevent removal of corrosive ions, allowing continued attack

Corrosion "focuses" on those spots, leading to faster disintegration in that region and failure at those points

Crevice Corrosion

 Shielded areas on metal surfaces are exposed to  corrosive environment

Ex. Bolt (there is a space the environment can get under the bolt above the metal- shielded)

Environment becomes stagnant

very narrow sites around a few thousands of inch, allow solution to enter but not leave

External surfaces protected because rate of oxygen reduction increases, leaving the external metal along

Usually occurs in chloride containing environments

Long incubation period of six months to one year before attack starts, but then accelerates after starting

It can be wielded to prevent crevice corrosion (wield the bolt down)

Avoid sharp corners and stagnate zones

Environment becomes stagnant- Crevice

Under a deposit, such as sand, dirt, solids out of solution, or under bolt heads and rivets, near gasket surfaces

Crevice

very narrow sites around a few thousands of inch, allow solution to enter but not leave

Crevice

Solution becomes depleted of oxygen, initially decreases the corrosion rate in the crevice

The metal is actively corroding, just very slowly, positive metal ions are collecting

At some point, those positive metal ions attract negative ions to migrate into this region

Negative ions increase, rate of metal dissolution increases, increasing the positive ions, increases the negative ion migration

Overall, the effect is to accelerate corrosion in an autocatalytic manner (once stated, cannot be stopped)

Pitting Corrosion

Localized corrosion resulting in holes in the metal

Small or large in diameter

Isolated (individual) or close together

Surface diameters is the same or less than the depth

Very destructive

Pitting Corrosion is Difficult

Difficult to detect because of small size AND that it is likely covered in corrosion products

Difficult to test for because of randomness in formation in same conditions, varying depths of the pits, and varying number of pits

Failure occurs with extreme suddenness- Pitting Corrosion

Minimal weight loss in entire structure but a stress concentration point was created, the "focused" stress to lead to failure

Pits grow

in the direction of gravity

Grow downwards from a horizontal surface

As with crevice corrosion, after starts, grows at increasing rate- Pitting Corrosion

Growth under the surface is more severe than surface damage indicates

Rapid dissolution of metal occurring- Type of anodic reaction- Pitting

(creates positive metal ions)

Migration of chloride ions into pit

Type of Anodic reaction- Pitting

counteract the positive metal charge

Switch to production of hydrogen ions

Type of anodic reaction- Pitting

(because chloride ions "left" the hydrogen behind to "join" the metal)

No oxygen reduction occurring in the pits

Oxygen reduction on metal surface

(with the abandoned hydrogen) surrounding pits protects other metal surfaces

Type of anodic reaction- Pitting

No mechanisms for start of pitting

 that adequately explains pitting nucleation (formation)

Believed it occurs when a spot momentarily has a higher dissolution of metal, causing chloride ions to migrate, stimulating more metal dissolution

type of anodic reaction- pitting

Pitting most commonly occurs in

chloride containing solutions

Also occur with other halide ions, like bromide and hypochlorite's

Not so corrosive with fluorides and iodides

Oxidizing metals in conjunction with chlorides are especially bad, CuCl2, FeCl3, HgCl2

Nonoxidizing metals are less aggressive but still corrosive

NaCl and CaCl2

type of anodic reaction- pitting

Associated with stagnent conditions

Allows chloride ions "settle", thereby increasing pitting

Type of anodic reaction

Stress Corrosion Cracking (SCC)

Cracking caused by presence of a tensile (load) stress and a corrosive medium

Have a stress concentration point, like a pit

No general pattern

Special Type Corrosion Fatigue

Stress concentration point- SCC

Corrosive media enters pit/point and as stress is applied, point opens allowing more media in, which reacts with the fresh metal

Stress corrosion cracking

as little as 10% of yield strength (which is already measured from 2% strain)

corrosion by-products can- SCC

increase stress within the crack by crating "wedges", causing the crack tip to grow faster

Accelerates crack growth beyond what corrosion alone would cause

No general pattern SCC

Occurs in aqueous media, as well as in liquid metals, salts, and nonaqueous inorganic liquids

Oxidizers do increase stress corrosion cracking

Accelerated by increasing temperature

Corrosion Fatigue

Instead of an applied tensile stress, this is a cyclically applied tensile stress (applied, released, applied, released, in a cycle)

 causes a reduction in fatigue due to the presence of a corrosive medium

 

Most pronounced at low stress frequencies

Have a greater contact time between the metal and the environment

Oxygen content, temperature, pH and solution affect corrosion fatigue

Hydrogen damaging

Mechanical Damage of a metal caused by the presence of, or interaction with, hydrogen

4 Types

1. Hydrogen Blistering

2. Hydrogen Embrittlement

3. Decarburization

4. Hydrogen Attack

Hydrogen Blistering

Hydrogen ions diffuse into metal and find a void

In void, combines with another hydrogen ion to form H2

The H2 can now NOT diffuse out

Concentration and pressure builds up, eventually resulting in rupture of the metal

The H2 pressure can build up to several hundred thousand atmospheres before rupture

Hydrogen Embrittlement

Hydrogen ions diffuse into metal and react with the metal

Forms a brittle, hydride compound

Destructive behavior is similar to stress-corrosion cracking, where hydride compound increase stress concentration, allows crack growth until failure

Main difference deals with the reactions

When specimen becomes more anodic, stress corrosion cracking (so more

M+ and e- produced)

Electrons flow FROM metal- Hydrogen embrittlement

increasing positive concentrations (M+), get stress corrosion cracking

Electron flow TO metal- Hydrogen Embrittlement

because of hydrogen bonding, hydrogen embrittlement

Decarburization

Removal of carbon from an alloy and occurs at high temperatures

Remove carbon precipitates, reduce the strength of the metal

Reaction of carbon with hydrogen forms methane

Cracking can occur when methane forms internally in voids

Chromium and molybdenum improve resistance because these carbides are more stable than iron carbides

Hydrogen Attack

Carbon in iron reacts with water vapor, forming carbon monoxide and hydrogen attack within the metal

Need high temperature for this to occur

Leaching

Removal of one element from a solid alloy by corrosion processes\

Is similar to pitting in that the overall dimension does not change appreciably

Removal of that element weakens the metal, causing it to become brittle

Often see holes AND color change, but if not examined closely, sudden failure can occur

Stagnant conditions favor leaching

Similar to pitting, small change greatly affect metal MAJOR difference- Leaching

with leaching, the holes can be anywhere, while with pitting, they only go with gravity

Dezincification- leaching- stagnant conditions

Removal of zinc from brass, leaving behind copper

Graphitization- leaching- stagnant conditions

Removal of iron (Fe) from gray cast iron, leaving behind graphite

Intergranular Corrosion

Usually occurs because boundary is slightly more reactive than grains, so usually, uniform corrosion dominates

Caused by impurities at the grain boundaries

Chromium is added to stainless steel for corrosion effects and mechanical behavior

Welds

A localized attack at adjacent to grain boundaries- Intergranular Corrosion

with relatively little corrosion of grains is intergranular corrosion

By just weakening the overall structure but over time, the grain boundary can be corrode enough that the grain falls out

Impurities at the grain boundaries

Enrichment of an alloying element

Depletion of an alloying element

Depletion of chromium near grain boundaries- Intergranular Corrosion

leads to intergranular corrosion

Chromium forms solid precipitates at grain, leaves adjacent areas (where there should be chromium) lower in chromium and more open to corrosion

Welds

is undisturbed

Because weld cools down quickly, elements cannot diffuse

Some distance from the weld (heat affected zone, HAZ) where heat transfer occurred, an alloying element precipitates out

When alloying element precipitates out, no corrosion protections exist

Stainless steels

1. High temperature solution treatment with quenching

High temperature ensures chromium carbide is dissolves

Quenching ensures rapid cooling so precipitates cannot form

1. Add elements to purposely form carbides

Titanium, Columbium, Tantalum have a strong affinity for carbon versus chromium

Combine with all carbons, preventing formation of chromium carbides

1. Lower carbon content to below 0.03%

Carbon formation is insufficient so intergranular precipitates cannot form

Difference between SS316 and SS316L

Erosion Corrosion

Acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface

Movement is rapid with mechanical wear and / or abrasion

Metal is removed as dissolved ions OR solid products are mechanically removed from surface

Characteristics of erosion corrosion

 grooves, gullies, waves, rounded holes, valleys

Formed in directional pattern

Occur in a relatively short amount of time

All metals are susceptible

Films that form an unreactive oxide, if the oxide is damaged, increased corrosion and increased byproducts (peeled aways oxide film)

Metal that are soft are easily worn mechanically and ball or peel

All environments are potential problems for erosion corrosion

Gases, Aqueous solution, organic systems, liquid metals

All types of equipment subjected to moving fluids are susceptible

Piping systems, mainly in bends, elbows, and tees

Valves, pumps, blowers, propellers, impellers, baffles, agitators, and agitated vessels, heat exchanger tubing

Factors of Erosion Corrosion

Surface films

Velocity

Turbulence

Impingement

Surface Films

protective oxide films formed protect against corrosion but not against wear, and can be delaminated because of wear

Velocity

increasing velocity, especially from increased rate of flow, increase erosion corrosion

Turbulence

Greater agitation of liquid at metal surface, more dissolved solid that can form

Impingement

Fluid forced to change direction

Fretting

Corrosion occurring at constant areas between materials under load subjected to vibration and slip- Knee replacement example

Pits or grooves surrounded by corrosion products in common

Characteristics of fretting

Can occur in the atmosphere or in media not usually corrosive

Engine components, automotive parts, bolted parts, implants

Destruction of metallic components through the production of oxide debris

Leads to seizing and galling because of the loss of tolerance and loosing

Loosening allows excessive strain, leads to fatigue fracture

To get fretting, you need an

 interface under load

1. Vibration or repeated relative motion

2. Load and motion must be sufficient to produce slip

slip

where you have small movement of a sliding nature

Only 10^-8 cm of relative motion is needed to cause fretting damage

Must be repeated- Continuous motion does not exhibit fretting

Ball Bearings

Ball Bearings

 In sealed environment with low viscous fluid, do not get fretting; get a piece of dirt/dust in fluid, causes bearing to move differently, get fretting

Cavitation Corrosion

Formation and collapse of vapor bubbles in liquid near a metal surface

Hydraulic turbines, ship propellers, pump impellers, other high-velocity liquid flow with pressure changes

Production and collapse of bubbles that produce shockwaves with pressure as high as 60,000 lb/in^2 (psi)

Leads to plastic (irreversible) damage

 

Resemble pitting (deep holes) but holes are closely spaced and surface is distinctly roughened