Lezione 17: Durability: oxidation, corrosion, degradation

Question 1

What is durability?

Answer 1

It’s a key material attribute, but it’s difficult to quantify and use for selection, because it’s a function of both the material and the environment.

Question 2

Which factors affect durability?

Answer 2

• Materials
• Application
• Environment
• Protection

Question 3

Talk about oxidation

Answer 3

The most stable form of most elements is an oxide. If the oxide adheres to the material, it gains weight in either a linear or parabolic way. If oxide is volatile, the sample loses weight in a linear way.

Question 4

Talk about flammability

Answer 4

Durability of polymers is dependant on their flammability

• Limiting oxygen index, LOI: the oxygen concentration in % required to maintain steady burning
• Underwriters laboratory rating (UL): response to a 1.6x127x12.7 mm polymer strip that is ignited while being held horizontally or vertically

Question 5

How does resistance to flammability work?

Answer 5

Combustion involves the reaction of free-radicals. Flame retardants work by either

• tying up the free radicals so they don’t react
• creating a protective layer between the solid polymer and the flammable gaseous products that enable combustion

Question 6

Talk about photo degradation

Answer 6

Polymers and elastomers age when exposed o light and oxygen, causing loss of strength, stiffness and toughness as well as discoloration. Additives (anti-oxidants, light stabilizers) are used to counteract this effect

Question 7

What are the mechanisms of oxidation?

Answer 7

Of metals: oxidation starts to be a problem at high Ts and wet environments.
When most metals are exposed to air, an ultra thin layer surface film of oxide forms: M+O = MO + energy
The oxide film separated metal from oxygen, to reach further either oxygen atoms must diffuse inward through the film or metal atoms must diffuse outwards.
Oxidation reaction occurs in 2 steps:
-Metal forms a ion and releases electrons M = M^2+ + 2e
-Electrons are absorbed by oxygen to give an oxygen ion O + 2e = O^2-
The first reaction occurs at the metal side of the oxide film, the oxygen reaction is on the other side. Either the metal ions and the electrons must diffuse out, or the oxygen and electron holes must diffuse in.
If the oxide film is compact and doesn’t crack, it protects the metal. This prevents further oxidation of the remaining metal. Although the free energy involves oxidation, the low speed of the process would result in the protection from further oxidation.
In the formed film is an electrical insulator, electrons cannot enter and the reaction continues only if the oxygen can diffuse into the metal directly.

Question 8

How can be assure resistance to oxidation?

Answer 8

Oxidation rates rise steeply with rising T: to enable materials to operate at high T precautions must be taken.

• Oxide coatings can be applied to metals like cast irons, steels, nickel alloys to prevent material degradation.
• Alloying elements such as chromium and silicon form oxides become passive and limit continued oxidation

Question 9

Talk about ions in solution and pH

Answer 9

Corrosion by acids and alkalis is an electro-chemical reaction. A chemical compound that dissociates in water increases either the hydrogen ion concentration or hydroxyl ion concentration: pH = -log10 [H+]
The pH level of an environment can initiate corrosion by stimulating a reaction in which a metal dissociates into a metal ion and releases electrons: M -> M^z + ze^-

Question 10

How does corrosion of metals occur?

Answer 10

Corrosion is an electrochemical reaction that always involves two processes:
-an oxidation process Me -> Me^z+ + ze- (metal dissolution)
-a reduction process (depending on the chemicals of the environment)
In presence of oxygen: 1/2O2 + H2O + 2e- -> 2OH-
In absence of oxygen: 2H+ + 2e- -> H2

A metal can oxidize depending on its intrinsic properties (oxidation potential) and depending on the reactivity of the environment (reduction potential of environment). In order to have corrosion, the reduction potential must be higher that the metal oxidation potential.

Question 11

What happens if a metal is places in salt water?

Answer 11

A metal placed in a conducting solution dissociates into ions and releases electrons. The anodic reaction governing this behavior is: Fe Fe++ + 2e-
The electrons accumulate on the iron giving it a negative charge that grows untile the electrostatic attractions pulls the metal ions back onto the surface. It reaches an equilibrium, indicated as E: it’s the reduction potential.

Question 12

What is the reduction potential?

Answer 12

Each metal has its own characteristic potential, called the standard reduction potential: the value is relative to a standard, the hydrogen standard potential.
If 2 metals are connected together in a cell, a potential difference appears between them. If a current is allowed, electrons flow from the anode to the cathode, causing corrosion of the anode. Example: iron and copper plates in copper sulfate solution
If the environment is changed from copper sulfate to water, there are no excess copper ions to plate out once a current is established: iron still corrodes, but cathodic reaction is now hydrolysis reaction: H2O + O + 2e- 2OH-
The reaction allows the oxygen to reach the copper.

Question 13

Is it necessary to have 2 metals to develop corrosion?

Answer 13

It’s not necessary to have 2 metals for a corrosion cell to develop: anodic and cathodic can take place on the same surface. An oxygen concentration gradient exists between metal at the surface of the water and the metal further immersed in the water. The potential difference between the hydrolysis reaction and the anodic reaction of iron ions dissociating drives the corrosion.

Question 14

Which different corrosion kinds exist?

Answer 14

Wet corrosion often occurs selectively instead of uniformly:

• Intergranular corrosion: occurs because grain boundaries have chemical properties that differ from those of the grain
• Pitting corrosion: preferential attack that occurs at breaks in the natural oxide on metals or at precipitate alloys in certain compounds
• Galvanic attack: appears in alloys with a 2-phase microstructure in which the 2 regions lie at different points on the reduction potential scale
• Stress corrosion cracking: accelerated corrosion, localized at cracks in loaded components
• Corrosion fatigue: accelerate rate at which fatigue cracks grow in a corrosive environment

Question 15

Which strategies can be used to fight corrosion?

Answer 15

• Good design: informed material, geometry and configuration choice
• Protective coatings: both active and passive
• Corrosion inhibitors: chemicals added to corrosive medium that retard the reaction
• Monitoring: protective maintenance or regular replacement

Question 16

List the do and don’ts for geometry and configuration in corrosion prevention

Answer 16

• Allow for uniform attack
• Avoid fluid trapping
• Suppress galvanic attack (welds should be on the dry side and bolts should be insulated with gaskets)
• Avoid crevices
• Consider cathodic protection
• Beware of stress corrosion and corrosion fatigue
• Design for inspection and maintenance

Question 17

What kinds of coatings exist?

Answer 17

• Passive coatings: they separate the material from the corrosive environment and are inherently corrosion resistant
• Active coatings: they sacrificially corrode to protect the metal
• Self-generated coatings: they rely on alloying in sufficient concentrations so that a protective film forms spontaneously

Coatings can be applied by_

• paint spraying
• hot dip galvanizing
• electroplating
• metal flame spraying
• polymer powder splaying
• enameling

Question 18

What are corrosion inhibitors?

Answer 18

They reduce the rate of attack when dissolved or dispersed in a corrosive medium. Some work by lowering the pH or by coating the part and suppress either the anodic or the cathodic step of the reaction. The choice of inhibitor depends on both the material and the environment.

Question 19

Metals are used in biomedical applications: what problems do they have?

Answer 19

Some metals in contact with skin can cause contact dermatitis, for example nickel is a well known allergen metal. In order to have interaction between the metal and the human body tissues, the metal has to be released in solution (sweat, plasma, blood) as metal ions Me^z+
The release of metal ions in solutions takes place through a corrosion process. Such process is the attitude of the metal to react with other species in order to reach a more stable equilibrium. Typically ceramics don’t corrode because they’re already oxidized or chemically stable compounds. So why are metals used for this applications? Because of their mechanical, physical, technological and aesthetic properties.

Question 20

Do metals that don’t corrode in presence of oxygen exist?

Answer 20

Yes, but they’re very few and they are very costly, they’re very dense and their mechanical performances make them inadequate for many structural application. They are: platinum, gold, iridium, palladium.

Question 21

Which factors influence corrosion?

Answer 21

First thing, is the difference between reduction potential of environment and oxidation potential al the metal. If ErEo then metals can corrode.
Corrosion is also related to the process velocity (kinetics).
Also, we must consider active and passive metals.
With active metals, the corrosion rate is high because of the high kinetics of the process.
If we have passive/active metals, some metals form an oxide layer of their surface. Here corrosion rate is very low, but still happens. For passive metals, low corrosion rate often has no or little effect on the component integrity and mechanical performance, but it can have important effect on biological environment, were only few ions can trigger reaction of the immunological system.
Also must pay attention to crevices because crevice corrosion can cause many problems even with stainless steels, for example by making the environment more aggressive.

Question 22

What is ion release effect?

Answer 22

Corrosion of active/passive metals alloys often don’t have any effect on the mechanical integrity of the components, even surface and finishing are not affected. But the release of ions in biological fluids or tissues can create inflammatory response, or allergic reactions. The allergic reaction is strictly connected with nickel content

Question 23

Talk about nickel in stainless steels

Answer 23

The most common and used metal alloys containing nickel are stainless steels. They are iron based alloys containing > 13% or chromium. Chromium is responsible for the passivation process (formation of protective chromium oxide layer). Other elements (Ni, Mo, N) are added to optimize properties.
Generally stainless steels are divided in 3 classes:
-austenitic
-ferritic
-martensitic
Reading them in the same order we have an increase in mechanical properties and a decrease in corrosion resistance.
Different types of stainless steels have been developed in time: for example P-558 is innovative because it has not nickel (<0,1), which is interesting for jewelry and biomedical application.