1. Corrosion Part 3 Corrosion Protection Methods
Corrosion Monitoring Devices
Corrosion Inspection Procedures
Environmental Factors

2. Corrosion Protection Methods
Slides 3 to 11

3. Maintaining pH Keep the pH of the water between 9 and 11.
This reduces acid or alkaline corrosion to a minimum.
This is usually done by adding sodium hydroxide to neutral water.
Alkaline amines such as cyclohexylamine are added since they are volatile and carry over with the steam to prevent acid corrosion in the condensate return system.

4. Mechanical Deaeration of O2
Mechanical Deaeration (removal of dissolved oxygen)
Remove dissolved oxygen from water.
This is done by heating the water to close to °C to drive off the dissolved gases in a deaerator.
Oxygen solubility decreases with temperature
This water must not be re-exposed to air or more oxygen will dissolve.

5. Chemical Deaeration of O2
Chemical Oxygen Scavengers
If an oxygen scavenger chemical is dissolved in the water it will react with the dissolved oxygen to reduce its concentration.
Sodium sulfite protects boilers.
2Na2SO3 + O2  Na2SO4
Carbohydrazide is volatile so it carries over with steam to protect condensate.
(N2H3)2CO + 2O2  2N2 + 3H2O + CO2

6. Galvanic Protection – Sacrificial Metals
If a metal higher than iron on the activity series such as zinc is bonded in good electrical contact to the iron, the zinc will corrode first, protecting the iron from corrosion.
This method of corrosion protection is called galvanic protection.
See the video referenced in the next slide

7. Video on Galvanic Cell Galvanic Cell Video
Both ZnSO4 and CuSO4 are 100% ionized
Zn, more anodic (reactive) than Cu, gives up its electrons to Cu
referring to the Activity Series for metals, Zn is more reactive than Cu
The zinc electrode corrodes over time

8. Corrosion Protection Methods – Corrosion Inhibitors
Protective Coatings
If the metal is coated so it is not exposed to water it will resist corrosion.
Aluminum oxide forms a durable oxide coat that protects aluminum from corrosion.
The chromium in stainless steel forms a durable oxide coat offering corrosion protection.
Magnetite (a form of iron oxide) protects iron.
Rust paints work by isolating the metal from water.

9. Corrosion Inhibitors Type Main Points Adsorbed Films
form a film, thin or thick, on the metal surface
first molecular layer of the film forms the strongest bond
may integrate into the corrosion product to form a composite film slowing further corrosion
can be anticorrosion paints or oil based films
Passivating Oxides
cause initial corrosion
the resulting corrosion product forms a protective oxide layer
pitting occurs where the passivating film does not cover the metal surface

10. Corrosion Inhibitors Type Main Points Cathodic Precipitates
Ionic salts form precipitates on cathodic areas to reduce the rate of corrosion
Effective even in low concentrations
Neutralizers
Used to increase pH by neutralizing acids and reducing the corrosion rate
Amines and ammonia a quite effective due to carry over

11. Corrosion Protect Methods – Impressed Current
If the iron is connected to the negative terminal of a DC power supply, this will supply electrons to the iron preventing its loss of electrons (oxidation).
A current of about 50 to 100 mA is required for each square meter of iron to be protected.

12. Corrosion Monitoring Devices
Slides 13 to 17

13. Corrosion Coupons
Corrosion coupons are uniform sized, weighed pieces of metal representative of system metals
Used to determine corrosion in a system over time:
system corrosiveness
material performance
inhibitor effectiveness
Applicable for gases, liquids, particulate flow

14. Electrical Resistance Probes
A conductive metal, exposed to corrosion, will have its surface area reduced, increasing its electrical resistance
Using a length of wire in a corrosive environment an electrical resistance probe can measure the change in resistance of the wire over time
This is used to determine the corrosion rate
Applicable for gases, liquids, particulate flow

15. Galvanic Probes Operates on the principle of the galvanic cell
Electrodes immerse in the fluid
The potential difference generates a current
The current relates to the rate of corrosion
Used in water injection systems to monitor for dissolved oxygen concentrations

16. Hydrogen Probes
Hydrogen probes measure the amount of hydrogen permeating through the metal
Two types mentioned
one design uses pressure of H2 gas generated by diffuse H atoms as a measure of the rate of corrosion
another uses an electrochemical cell that attaches to the surface of a pipe
current is generated to oxidize surface hydrogen
corrosion rate is proportional to the current produced.
hydrogen induced SCC

17. Corrosion Inspection Procedures
Slides 19 to 23

18. Direct Observation All observations need to be properly documented
start with a baseline condition
Include:
photographs
date, time
written description
assessment of any damage

19. Magnetic Particle Inspection
Used for ferromagnetic metals (can be magnetized) such as carbon steel
magnetize the area of interest
sprinkle on iron powder or filings
filings indicate the location and size of cracks in the metal

20. Dye Penetrant Inspection
Indicates surface flaws only
Low cost method of inspection
Kit includes:
surface cleaner
indicator dye
a developer which draws the dye back out of the surface defect indicating its location

21. Ultrasonic Inspection
Uses ultrasound to detect flaws on or beneath the metal surface
based on sound propagation and reflection
Used to inspect welds

22. Radiography
In essence an “x-ray” of a horizontal cross-section of pipe
x-rays or gamma rays are used
corroded areas show up as dark spots on the “x-ray” film

23. Environmental Factors
Slides 25 to 32

24. Fluid Velocities Velocity is too fast Velocity is too slow
wears away protective, anti-corrosion film on metal surface
Velocity is too slow
particles settle out of fluid on the metal surface preventing anti-corrosion chemical treatment
sites of particle accumulation subject to corrosion

25. pH pH < 9 promotes corrosion
in anodic areas low pH corrodes protective metal oxide films
due to oxidation of iron
in cathodic areas low pH produces OH−
results in Fe(OH)2
as acidity increases the rate of corrosion increases

26. Temperature
Corrosion related reactions become more vigorous as temperature increases
warmer areas on a piece of metal become anodic
subject to corrosion

27. Oxygen Content
Corrosion rates increase as oxygen levels around the cathode area increase
electrons travelling from the anodic to cathodic areas combine with oxygen at the cathode’s surface

28. Microorganisms
Can change the amount of oxygen in the water affecting corrosion rates
can increase rate by supplying oxygen to cathodic areas of the system
can form deposits on metal surfaces with other matter
an anodic site develops
results in pitting

29. Dissolved Solids and Gases
Dissolved solids result in:
interference of formation of corrosion inhibitor films
increasing the fluids ability to conduct current which increases corrosion
Dissolved gases result in:
O2 combines electrons at the cathode
CO2 H2S increases acidity

30. Cooling Water Systems
Due to design and construction these systems are a galvanic cell type of corrosion
cooling water is oxygen rich
cooling water high is dissolved solids
equipment temperature differences produce anodic and cathodic areas
water can become acidic
biological growth can occur