Past Paper Flashcards
(8 cards)
State the four components that comprise a wet corrosion cell. What happens to
the corrosion reaction if one of these components is removed?
Wet corrosion comprises four basic electrochemical
units.
1. Anode - Metal oxidation
2. Cathode - reduction reactions
3. Electron flow from anode to cathode
4. An electrolyte or aqueous medium (water salt etc.)
To stop wet corrosion happening we must interrupt one
of these four components of our corrosion cell.
Identify and explain a potential corrosion mechanism and suggest suitable
countermeasures for the following cases. Consider the MATERIALS,
ENVIRONMENT and DESIGN in the system to help your answer. (Each answer
should be a maximum of 150 words)
A 304 grade stainless-steel turbine blade in a power plant failed
catastrophically when spinning under load. The water used to generate the
steam that turned the turbine was found to be contaminated with traces of
chloride ions.
i) 304 Stainless Steel Turbine Blade + Chloride-Contaminated Steam
What happened:
Chloride ions in the steam attacked weak spots in the stainless steel, especially under stress from spinning. This caused small cracks to grow quickly, leading to sudden failure. This process is called stress corrosion cracking.
Why:
304 stainless steel is not very resistant to chloride in high-temperature, high-pressure environments. The spinning motion adds stress, which makes it worse.
Countermeasures:
Material: Use a more resistant alloy like 316 stainless steel or duplex stainless steel, which can better handle chlorides.
Environment: Improve water quality—remove chloride ions from steam.
Design: Reduce stress concentrations by avoiding sharp corners or poorly finished surfaces.
Identify and explain a potential corrosion mechanism and suggest suitable
countermeasures for the following cases. Consider the MATERIALS,
ENVIRONMENT and DESIGN in the system to help your answer. (Each answer
should be a maximum of 150 words)
70/30 brass bolts that were used to secure lifeboat winches on a ship were
found to rapidly fail after time at sea. On inspection, the failed bolts had a
surface that was highly porous.
ii) 70/30 Brass Bolts on Ship Lifeboat Winches
What happened:
The brass bolts lost zinc from their surface over time in salty sea air, leaving behind a porous, weak copper layer. This is called dezincification.
Why:
70/30 brass (70% copper, 30% zinc) is vulnerable in salt-rich environments. The salty air and sea spray accelerated the attack.
Countermeasures:
Material: Use dezincification-resistant brass (DZR brass) or stainless steel bolts.
Environment: Apply protective coatings or use sealed enclosures.
Design: Use drainage and shielding to reduce exposure to saltwater.
Identify and explain a potential corrosion mechanism and suggest suitable
countermeasures for the following cases. Consider the MATERIALS,
ENVIRONMENT and DESIGN in the system to help your answer. (Each answer
should be a maximum of 150 words)
A large penetrative corrosion defect was found in a 316L stainless steel heat
exchanger panel. The hole was located underneath a rubber gasket that
separated the panels of the heat exchanger. The heat exchanger had been
exposed for many hours to water that contained sodium hypochlorite (bleach)
to disinfect the system.
iii) 316L Stainless Steel Heat Exchanger + Sodium Hypochlorite
What happened:
Bleach trapped under the rubber gasket attacked the steel over time, creating a deep hole. This is crevice corrosion, which happens in tight, moist spaces with chemicals present.
Why:
Even though 316L is normally resistant to bleach, it can corrode in crevices where oxygen is low and chemicals build up.
Countermeasures:
Material: Use super austenitic stainless steel or titanium, which are more resistant to bleach.
Environment: Reduce or control the use of strong chemicals like sodium hypochlorite.
Design: Avoid tight crevices; use non-absorbent gaskets and ensure good cleaning and drying access.
Identify and explain a potential corrosion mechanism and suggest suitable
countermeasures for the following cases. Consider the MATERIALS,
ENVIRONMENT and DESIGN in the system to help your answer. (Each answer
should be a maximum of 150 words)
A stainless-steel pipe in a food processing plant was found to fail after
corrosive attack at a welded joint after exposure to fluid containing brine.
iv) Stainless Steel Pipe Weld Joint in Brine
What happened:
The weld joint corroded more quickly than the rest of the pipe after exposure to salty fluid (brine). This is likely weld decay or intergranular corrosion, where the welding process weakens the steel along grain boundaries.
Why:
Improper welding or lack of post-weld treatment can leave the area vulnerable, especially in salty environments.
Countermeasures:
Material: Use low-carbon stainless steel (like 316L) and proper welding rods.
Environment: Limit exposure time to brine or rinse thoroughly after cleaning.
Design: Apply post-weld heat treatment or use automatic welding with good shielding gas to avoid defects.
Corrosion inhibitors are chemical additions made to systems to reduce the corrosion
rate. Discuss the use and theory of corrosion inhibitors as a means of controlling
corrosion with regards to:
i) Anodic inhibition through precipitation control
For i) and ii) discuss some advantages and
disadvantages of each type of inhibitor.
In your answers draw schematic Evans diagrams, where applicable, to explain the
inhibitors mechanisms of operation.
i) Anodic Inhibition through Precipitation Control (10 marks)
Theory:
Anodic inhibitors work by forming a protective oxide film on the metal surface. This slows or stops the anodic reaction (where metal dissolves). These inhibitors often cause controlled precipitation of oxides/hydroxides at the anode.
Common examples:
Chromates
Phosphates
Molybdates
Mechanism (Evans Diagram):
The anodic polarization curve shifts to the left, increasing the corrosion potential (Ecorr) and decreasing the corrosion current (Icorr).
🔽 This reduces the rate of metal dissolution.
Advantages:
Highly effective at low concentrations
Can form strong, durable protective layers
Disadvantages:
Risk of pitting or localized corrosion if the layer is incomplete
Some anodic inhibitors (e.g. chromates) are toxic and environmentally hazardous
Can cause overprotection, making metals more brittle
Corrosion inhibitors are chemical additions made to systems to reduce the corrosion
rate. Discuss the use and theory of corrosion inhibitors as a means of controlling
corrosion with regards to:
ii) Cathodic precipitation and poisoning inhibitors
For i) and ii) discuss some advantages and
disadvantages of each type of inhibitor.
In your answers draw schematic Evans diagrams, where applicable, to explain the
inhibitors mechanisms of operation.
ii) Cathodic Precipitation and Poisoning Inhibitors (10 marks)
Theory:
These inhibitors interfere with the cathodic reaction (usually oxygen reduction or hydrogen evolution). They work in two ways:
Precipitation inhibitors (e.g. calcium carbonate or zinc salts) form a barrier on the cathodic sites.
Poisoning inhibitors (e.g. arsenic, antimony compounds) disrupt the reaction mechanism at the cathode.
Mechanism (Evans Diagram):
The cathodic curve shifts to the right, lowering Icorr. The reaction becomes slower at the cathode.
Advantages:
Good for systems where cathodic reaction is dominant
Precipitated films can cover large areas effectively
Disadvantages:
Some (like arsenic) are toxic and hazardous
Precipitation can clog or foul equipment
Less effective if water chemistry changes (pH, hardness)
Corrosion inhibitors are chemical additions made to systems to reduce the corrosion
rate. Discuss the use and theory of corrosion inhibitors as a means of controlling
corrosion with regards to:
iii) Adsorption inhibitors
For i) and ii) discuss some advantages and
disadvantages of each type of inhibitor.
In your answers draw schematic Evans diagrams, where applicable, to explain the
inhibitors mechanisms of operation.
iii) Adsorption Inhibitors (5 marks)
Theory:
These inhibitors adsorb onto the entire metal surface, forming a thin protective layer. They block both anodic and cathodic reactions. Often organic compounds like amines, azoles, or fatty acid salts.
Mechanism:
They physically or chemically bond to the metal, forming a film that prevents water, oxygen, or ions from contacting the surface.
Common in:
Closed-loop systems
Oil and gas pipelines
Industrial cooling water
Advantages:
Simple to apply
Usually less toxic
Effective across different metals
Disadvantages:
Can be sensitive to temperature or flow rate
May need frequent reapplication
