An engineer bolts a small stainless steel fastener into a large aluminum structural panel for use in a marine environment. The galvanic series shows aluminum is more active (anodic) than stainless steel. What does the electrochemical circuit model predict?
AThe stainless steel bolt corrodes rapidly because it is the smaller component and more exposed
BBoth metals corrode at equal rates because they share the same electrolyte (seawater)
CThe aluminum near the bolt corrodes rapidly — it is the small anodic area coupled to a large cathodic area, concentrating all corrosion current at the contact zone
DNo significant corrosion occurs because aluminum's passive oxide layer protects it in marine environments
This is a dangerous area ratio configuration: the small aluminum anode (the contact area around each bolt) is coupled to a large stainless steel cathode. All the corrosion current concentrates on the small anodic area, causing rapid localized pitting of the aluminum. Option D is the common misconception — aluminum's passive layer is disrupted by chloride ions in seawater, which is why 'aluminum is protected by passivation' is insufficient reasoning in marine environments.
Question 2 Multiple Choice
Why does cathodic protection prevent corrosion of a buried steel pipeline?
AIt coats the pipeline with a passive chromium oxide layer similar to stainless steel
BIt eliminates the electrolyte (moist soil) surrounding the pipeline, breaking the electrochemical circuit
CIt makes the pipeline the cathode — by connecting it to a sacrificial anode or impressing current — so metal dissolution at the surface cannot occur
DIt increases the pipeline's electrical resistance, reducing the corrosion current to negligible levels
Corrosion requires anodic dissolution (metal → metal ions + electrons) at the corroding surface. Cathodic protection forces the pipeline to act as the cathode, where reduction reactions (not oxidation) occur. At a cathode, metal does not dissolve. Either a sacrificial anode (e.g., magnesium, which is more active and corrodes preferentially) or an impressed current (external power supply forcing electrons into the pipeline) achieves this. The electrochemical driving force for dissolution is removed because the pipeline is no longer the anode.
Question 3 True / False
Stainless steel is corrosion-resistant because the iron and chromium in the alloy are inherently noble metals that do not react with water or oxygen under normal conditions.
TTrue
FFalse
Answer: False
Stainless steel's corrosion resistance comes entirely from its passive layer — a thin, adherent Cr₂O₃ film that forms spontaneously and blocks further oxidation. Iron and steel are not inherently noble; plain steel corrodes readily. In chloride-rich environments (seawater, road salt), chloride ions can penetrate the passive film at local defects, initiating pitting corrosion that grows autocatalytically. The common belief that 'stainless steel doesn't corrode' is a dangerous oversimplification that has caused failures in marine and chemical processing applications.
Question 4 True / False
In galvanic corrosion, the rate at which the anodic metal dissolves depends not only on the electrochemical potential difference between the two metals, but also critically on the relative surface areas of the anode and cathode.
TTrue
FFalse
Answer: True
Area ratio is a key engineering variable in galvanic corrosion. A small anode coupled to a large cathode concentrates all the corrosion current on the small anode — intense localized attack. A large anode coupled to a small cathode spreads the same current over a large area — slow, diffuse attack. This is why the joint design matters as much as alloy selection: stainless steel fasteners in an aluminum panel create a large-cathode/small-anode configuration that accelerates aluminum failure at each contact point.
Question 5 Short Answer
A protective coating on a steel pipeline develops a small pinhole defect. Explain, using the electrochemical circuit model, why this defect can cause more severe corrosion at that spot than if the entire pipeline were left uncoated.
Think about your answer, then reveal below.
Model answer: The coating isolates the vast majority of the steel surface from the electrolyte, making it effectively cathodic (no anodic reactions possible). The tiny exposed steel at the pinhole becomes the only anode — a small anodic area coupled to a very large effective cathodic area (the entire coated surface that can still participate in the cathodic reduction circuit). All corrosion current concentrates at the defect, producing rapid deep pitting rather than the gradual uniform attack that would occur on a fully uncoated surface.
This 'holiday problem' is a well-known failure mode in coated pipeline systems. The coating paradoxically worsens attack at any breach because it creates an extreme area ratio: one tiny anode, enormous cathode. This is why coating integrity monitoring is critical for cathodically protected pipelines, and why cathodic protection is typically used alongside coatings rather than as an alternative — the two strategies address each other's failure modes.