Corrosion and protection of steel structures Corrosion:

Question 1

How much does corrosion cost per year to the US economy?

Answer 1

$ 276 billion

Question 2

How much of an industrialised country GDP is spent on corrosion?

Answer 2

3.5%

Question 3

How much of the annual world wide production of iron and steel is used to replace material lost through corrosion?

Answer 3

15%

Question 4

What is corrosion?

Answer 4

Degradation in properties and appearance and loss of material due to dissolution

Question 5

Where does it occur?

Answer 5

Surface of material

Question 6

What are the corrosion costs from Drinking Water and Sewer Systems?

Answer 6

$36 billion

Question 7

What is the second biggest source for corrosion costs?

Answer 7

Motor vehicles

Question 8

What are the direct losses from corrosion on the economy?

Answer 8

• Component replacement
• Painting and other preventative measures
• Use of expensive corrosion-resistant materials

Question 9

What are the indirect losses from corrosion on the economy?

Answer 9

• Plant shutdown
• Loss of product e.g. in pipes
• Loss of efficiency
• Contamination of product

Question 10

What effect is the corrosion of metals?

Answer 10

Electrochemical effect

Question 11

What happens in the chemical reaction?

Answer 11

• Electrons are transferred
• REDOX reaction

Question 12

What occurs at the anode during metal corrosion?

Answer 12

During metal corrosion, metal atoms are oxidized at the anode, meaning they form positive ions and release electrons. This site is where the anodic reaction occurs and where corrosion typically happens.

Question 13

In the provided examples, what happens to iron (Fe) when it oxidizes?

Answer 13

When iron (Fe) oxidizes, it forms Fe2+ ions and releases 2 electrons (2e-)

Question 14

How does aluminum (Al) react in an oxidation reaction?

Answer 14

In an oxidation reaction, aluminum (Al) forms Al3+ ions and releases 3 electrons (3e-)

Question 15

What is the oxidation state and number of electrons released when zinc (Zn) corrodes?

Answer 15

When zinc (Zn) corrodes, it forms Zn2+ ions and releases 2 electrons (2e-)

Question 16

What happens during the corrosion of Zn in an acidic solution?

Answer 16

During the corrosion of Zn in an acidic solution, Zn corrodes into Zn2+ at the anode, and electrons are transferred to the cathode forming hydrogen gas, with H+ being reduced to hydrogen at the cathode.

Question 17

What does the nature of the cathodic reaction depend on?

Answer 17

Environment

Question 18

When electrons formed what are he transferred to?

Answer 18

Electrons formed are transferred to another chemical species

Question 19

Where is the H+ ions reduced?

Answer 19

Cathode

Question 20

Where do electrons go to form hydrogen gas?

Answer 20

Cathode

Question 21

How is iron oxide formed and why is it energetically favorable for iron to form rust?

Answer 21

Iron oxide, or rust, is formed when iron reacts with oxygen in wet conditions. It is energetically favorable for iron to form rust because iron found as iron oxide in iron ore tends to revert to the oxide as part of an electrochemical process.

Question 22

What happens to iron extracted?

Answer 22

Tends to reform the oxide in an electrochemical process

Question 23

What initiates the process of rust formation on iron?

Answer 23

Rust formation is initiated when oxidizing iron supplies electrons at the edge of the water droplet to reduce oxygen from the air.

Question 24

What role does the iron surface inside the droplet play in rust formation?

Answer 24

The iron surface inside the droplet acts as the anode, where oxidation occurs as iron loses electrons.

Question 25

What happens to the electrons generated at the anode during the rust formation process?

Answer 25

Electrons generated at the anode move through the iron to the outside of the water droplet, where reduction occurs at the cathode.

Question 26

Describe the process that occurs within the droplet leading to the precipitation of iron(II) hydroxide.

Answer 26

Inside the droplet, hydroxide ions can move to react with the iron(II) ions coming from the oxidation region, resulting in the precipitation of iron(II) hydroxide.

Question 27

How is rust produced from iron(II) hydroxide?

Answer 27

Rust is produced by the further oxidation of the precipitated iron(II) hydroxide.

Question 28

What kind of process drives the formation of rust?

Answer 28

The formation of rust is driven by an electrochemical process.

Question 29

What command is rust?

Answer 29

• FeOH

Question 30

What does the Galvanic Series indicate?

Answer 30

The Galvanic Series gives the relative reactivity of some common materials in seawater, listing metals in decreasing order of their tendency to corrode, with the most noble (least reactive) at the top.

Question 31

What are some of the most cathodic (noble) materials in the Galvanic Series?

Answer 31

The most cathodic (noble) materials in the Galvanic Series are Platinum, Gold, and Graphite.

Question 32

Which metals are considered the most anodic and likely to corrode in the Galvanic Series?

Answer 32

The most anodic metals, which are most likely to corrode, are Zinc, Magnesium, and Cadmium.

Question 33

How can corrosion be avoided in materials?

Answer 33

Corrosion can be avoided by using materials resistant to corrosion, providing a physical barrier like paint to the surface of the component, using galvanic or cathodic protection, and galvanizing.

Question 34

What is the challenge associated with using coatings such as paint for corrosion protection?

Answer 34

It is difficult to ensure that coatings like paint will last over time, especially in structures.

Question 35

What does galvanic protection involve?

Answer 35

Galvanic protection involves using a galvanic couple where the metal to be protected is connected to a more reactive metal, such as Magnesium or Zinc, to prevent its corrosion.

Question 36

What is cathodic protection and how does it work?

Answer 36

Cathodic protection is a method that supplies electrons from an external source to force the oxidation reaction to reverse, thereby protecting the metal from corrosion.

Question 37

What is the purpose of a polymer-coated steel pipe, and what does it typically consist of?

Answer 37

The purpose of coating a steel pipe with polymers, such as HDPE and an epoxy layer, is to protect it against corrosion. This method is quite expensive but effective.

Question 38

What is the purpose of connecting a steel pipe to an active metal like magnesium in galvanic protection?

Answer 38

The purpose is to form a sacrificial anode with magnesium, which is oxidized in preference to the iron in the steel pipe, protecting the pipe from corrosion.

Question 39

Why is magnesium used in galvanic protection for steel pipes?

Answer 39

Magnesium is used because it will act as the anode in the FeMg voltaic cell, oxidizing in preference to the steel, and it’s easier to replace the magnesium rod than the steel pipe.

Question 40

What role does the steel pipe play in the FeMg voltaic cell in galvanic protection?

Answer 40

In the FeMg voltaic cell, the steel pipe is involved in a cathodic reaction, which means it is protected from oxidation.

Question 41

How is galvanising achieved, and what is its benefit?

Answer 41

Galvanising is achieved by applying a layer of zinc to steel through hot dipping, creating a galvanic circuit to the steel. It has a low carbon footprint and can be recycled.

Question 42

Why is zinc used for hot dip galvanising coatings, and what happens when it corrodes?

Answer 42

Zinc is used for hot dip galvanising coatings because it corrodes in preference to steel and sacrifices itself, which is beneficial since zinc is abundant. The corrosion products from zinc are deposited on the steel, resealing it from the atmosphere and stopping further corrosion.

Question 43

What happens if a galvanised coating is damaged and reveals the bare steel?

Answer 43

If the galvanised coating is damaged, the steel can still be protected because the zinc provides both barrier and sacrificial protection.

Question 44

How does the rate of weathering of zinc in a galvanised coating benefit the steel it protects?

Answer 44

The zinc in a galvanised coating weathers at a very slow rate, which provides a long and predictable life for the steel it protects.

Question 45

What is one of the sacrificial protection benefits of galvanised coatings on steel?

Answer 45

Galvanised coatings provide sacrificial protection to small areas of steel exposed through drilling, cutting, or accidental damage, as the coating corrodes preferentially.

Question 46

How does galvanising prevent sideways creep on larger damaged areas of steel?

Answer 46

If the damaged area on the steel is larger, the sacrificial protection offered by the galvanising prevents sideways creep which can undermine coatings.

Question 47

What is the significance of the coating toughness in galvanised steel?

Answer 47

The coating toughness is significant because the galvanised coating is bonded metallurgically to the steel, which makes it very durable.

Question 48

Why is galvanising considered to have the lowest lifetime cost?

Answer 48

Galvanising is considered to have the lowest lifetime cost because of its durability and the fact that the protective coating lasts more than 40 years, reducing the need for frequent maintenance or replacement.

Question 49

What makes the inspection of galvanised steel easy?

Answer 49

The ease of inspection for galvanised steel is due to the obvious visual signs of wear or damage to the zinc coating, allowing for straightforward assessments of the coating’s integrity.

Question 50

What is the main principle behind cathodic protection?

Answer 50

Cathodic protection works by redirecting current to flow from a sacrificial anode to the soil-water electrolyte, instead of from an anode area on a pipeline or other metallic structure, thereby protecting the structure from corrosion.

Question 51

What is the role of the protective anode in cathodic protection?

Answer 51

The role of the protective anode in cathodic protection is to corrode in place of the metallic object it is designed to protect.

Question 52

How is the negative end of the power supply used in cathodic protection?

Answer 52

The negative end of the power supply is attached to the metal to be protected, ensuring that the metal acts as the cathode and does not corrode.

Question 53

Where is the positive terminal of the power supply connected in a cathodic protection system?

Answer 53

The positive terminal is attached to an inert (usually graphite) protective anode that is buried in the soil, which is where the corrosion occurs.

Question 54

What happens to the current in a cathodic protection system for buried pipelines?

Answer 54

In a cathodic protection system for buried pipelines, the current flows from the protective anode through the soil and towards the pipeline, which is protected as the cathode.