actual past paper Flashcards
(20 cards)
wet corrosion cell components
anode - where metal loses electrons aka oxidation
cathode - where reduction happens aka gains electrons
electrolyte - conducts ions, usually water with dissolved salts
metallic path - allows electron flow between anode and cathode
if any component is removed, the corrosion reaction stops since the circuit is incomplete
Identify and explain a potential corrosion mechanism and suggest suitable countermeasures for the following case. 304 stainless steel turbine blade + chloride
mechanism - stress corrosion cracking due to chloride ions and tensile stress.
countermeasures - use a higher grade steel, remove chlorides from steam, and reduce stress concentrators. maybe add a surface polish.
Identify and explain a potential corrosion mechanism and suggest suitable countermeasures for the following case. 70/30 brass bolts on ship
mechanism - dezincification in seawater means porous copper layer
countermeasures - use dezincification-resistant alloys, protective coatings, and avoid brass in marine load-bearing roles.
Identify and explain a potential corrosion mechanism and suggest suitable countermeasures for the following case. 316L SS heat exchanger + bleach NaOCl
mechanism - crevice corrosion under gasket
countermeasures - improve grade or use titanium, avoid oxidizing cleaners like bleach, minimize crevices, better gasket sealing
Identify and explain a potential corrosion mechanism and suggest suitable countermeasures for the following case. welded joint in SS pipe + brine
mechanism - weld decay leads to sensitization leads to intergranular corrosion
countermeasures - use low carbon SS like 316L, flush to remove salt, post-weld heat treatment or TIG welding
explain anodic inhibitors (precipitation control)
sodium chromate promotes the formation of a passive oxide film on the anodic areas of a metal surface. this blocks further metal dissolution (oxidation) at the anode
is often used for steel in cooling water systems
think of it as protecting the areas where metal wants to give up electrons
pros - effective, stable film
cons - can cause pitting if layer breaks
explain cathodic inhibitors (precipitation and poisoning)
calcium carbonate reduces corrosion by slowing the cathodic reaction, usually the reduction of oxygen or hydrogen ion.
it can precipitate on cathodic sites, creating a barrier that makes it harder for electrons to be consumed
common in boiler and water systems, where scaling can be protective
this type of inhibitor defends the spots where electrons would normally be used up in the corrosion reaction
pros - can suppress hydrogen embrittlement
cons - may be toxic or form weak films
explain adsorption inhibitors
tannins are organic compounds that physically adsorb onto the metal surface
they form a thin protective film that blocks corrosive species (like water, oxygen, chloride ions) from reaching the metal
often used in oil pipelines, closed-loop water systems, and conservation of historical iron artifacts
these inhibitors act like a chemical cling wrap, coating the metal and sealing it off from the environment
pros - versatile and widely applicable
cons - limited protection in aggressive conditions, continuous reapplication
With the aid of an appropriate schematic diagram, describe a type of experiment
which could be used to measure the oxidation kinetics of metallic nickel heated to
800C in an oxygen environment. As part of your answer, include a sketch of the
time-dependent data you expect to obtain
use thermogravimetric analysis (TGA) - high-temperature oxidation test used to measure how fast a metal oxidizes over time
plot weight gain vs time
expect parabolic growth which means the oxide layer is protective. as the oxide grows, it slows down oxygen diffusion so the oxidation rate drops
oxide layer is protective - slows down oxidation
oxide layer is non-protective - allows continued corrosion
Briefly explain how Ni-Cr alloys with a Cr composition of ≥ 5 % w/ have a significantly greater resistance than pure Ni to high temperature oxidation
nickel on its own forms a non-protective oxide layer at high temps. when at least 5% chromium is added, it forms a stable and dense chromia scale on the surface. this layer is slow-growing and diffusion resistant meaning blocks both metal and oxygen ion movement
In relation to the formation of a metal oxide layer on the surface of a metal, what is meant by the Pilling-Bedworth ratio
dimensionless number used to evaluate how protective an oxide layer is when a metal oxidizes
Molar mass of oxide times density of metal over molar mass of metal times density of oxide
Using appropriate examples, explain how the Pilling Bedworth ratio can be used to evaluate the protective nature of an oxide film against further corrosion
if pbr is less than one, the oxide film is non-protective and cracks
if pbr is between 1 and 2, the oxide film is protective. for cobalt forming CoO pbr is 1.76 meaning the oxide layer is dense and adherent enough to protect the underlying metal from further oxidation
if pbr is greater than 2 the oxide is too bulky which can flake off and be non-protective
oxidation of iron at 400c
400-main oxide fe3o4 (magnetite) forms slowly. Oxide layer is thin, adherent, and compact. Oxidation shows a linear relationship between weight gain and time, but eventually transitions to parabolic as oxide thickens. Rate determining step – at this temp, electron transfer and oxygen adsorption at the surface are relatively slow. Diffusion of metal cations fe2+ through the growing oxide becomes rate-limiting over time. Interfacial reaction – fe2+ and electrons move outward through the oxide to meet incoming oxygen
oxidation of iron at 750c
750 – multi-layer oxide forms, scale becomes thicker, possibly non-adherent with more defects. Formation of multiple oxides is due to higher oxygen mobility and thermodynamic stability ranges. The weight gain vs time relationship is parabolic which reflects diffusion-controlled growth over time. Rate determining step – diffusion of fe2+ cations through the scale is dominant. Higher temp increases ionic mobility, accelerating oxidation. Interfacial reaction – iron ions diffuse outward and react with oxygen at the surface
what is meant by rate determining step
The rate-determining step is the slowest step in a multi-step process that controls how fast the overall reaction or process occurs.
What type of material characterization or analytical technique would be suitable
to determine the thickness of an oxide layer
ellipsometry, SEM cross-sections
What type of material characterization or analytical technique would be suitable
to determine the morphology of an oxide layer
SEM - scanning electron microscopy
What type of material characterization or analytical technique would be suitable
to determine the identification of different phases present in an oxide layer on a pure metal
X-ray diffraction (XRD)
What type of material characterization or analytical technique would be suitable
to determine the chemical composition of an oxide layer formed on an alloy surface
EDX - energy dispersive z-ray spectroscopy, XPS - x-ray photoelectron spectroscopy
