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CORROSION EXAM 2 2026 CORROSION
SCIENCE AND MATERIAL PROTECTION
ASSESSMENT TEST PAPER QUESTIONS
SOLUTIONS GRADED A+
◉ Ways to alter the environment to impact the rate of corrosion: Answer: - change chemical concentration
- minimize contaiminants
- change the concentration of dissolved oxygen
- change the velocity
- temperature ◉ Example of how chemical concentration affects corrosion rate: Answer: steel in H2SO4 will cause an increase then decrease in corrosion, while Pb in H2SO4 will cause only an increase in corrosion rate ◉ How can minimizing contaminants help with corrosion? Answer: small amounts of some contaminants can greatly impact corrosion rates such as:
- Cl ions in contact with SS, can lead to breakdown of passive layer in SS such as pitting and SCC
- Atmospheric corrosion, industrial atmospheres contain higher quantities of SO2 due to burning of sulfur laden fuels, under humid conditions metal oxide catalyzes ◉ Example of how changing the concentration of dissolved oxygen can affect corrosion: Answer: - for mild steels: corrosion rates tend to increase with an increase in oxygen concentration
- for stainless steels: corrosion rates tend to decrease with an increase in oxygen concentration (O2 is required to maintain passive layer) ◉ How can a change in velocity affect the corrosion rate? Answer: - velocity can affect corrosion rate for corrosion mechanisms controlled by concentration polarization
- velocities that exceed the "critical velocity" for a particular active passive metal can damage the passive layer ◉ How can temperature affect corrosion rate? Answer: - operating at lower temps may decrease corrosion rates in many circumstances ◉ What is the general rule for temp when it comes to corrosion?
◉ Types of Corrosion inhibitors: Answer: 1. Passivators
- Barrier Inhibitors
- Cathodic Poisons
- Scavengers
- Neutralizers ◉ What is an example of a passivator? Answer: anodic inhibitors ◉ What can passivators do? Answer: - shift the corrosion potential of the anode in a more noble (+) direction to cause passivation
- stabilize an existing passive film
- aid repassivation if the film is damaged ◉ Why can passivators be dangerous? Answer: dangerous if not used in sufficient quantity, all attack will concentrate on the non-passive regions ◉ What will happen to a small region of uncoated (passivator)?
Answer: - will become more anodic
- accelerate corrosion ◉ Types of Passivators: Answer: 1. Direct passivators (oxidizers)
- Indirect passivators ◉ How do direct passivators work? Answer: - contain anions that react with the metal surface
- these are incorporated in the passive film
- they act to form or strengthen a passive layer ◉ What are the best direct passivators? Answer: - best oxidizers: CrO4^-3, NO2^-
- less effective, require O2 in the system: MoO4^-2, WO4^- 2 ◉ What do indirect passivators do? Answer: they improve the adsorption of dissolved O2 onto the metal surface (for metal that form a stable protective oxide layer), there must be O2 in the system for indirect passivators ◉ How do indirect passivators work?
◉ What are organic adsorption inhibitors? Answer: - coat the surface of the metal with an oily layer
- will adsorb on any surface (metal, porous, rust, dirt)
- most commonly used in acidic environments ◉ Examples of Organic adsorption inhibitors: Answer: - Amines (R - NH2)
- Carboxyls (R - COOH) ◉ What are inorganic precipitate inhibitors? Answer: - react with the environment to form an insoluble precipitate on either the anode, cathode, or both ◉ Examples of Inorganic precipitate inhibitors: Answer: phosphates (PO4^-3) will precipitate FeHPO4 and FePO on steel ◉ How do Cathodic Poisons work? Answer: - work by slowing the hydrogen cathodic reaction
- tend to inhibit the ability of H to form H2 gas
- can lead to hydrogen embrittlement, hydrogen induced cracking, and hydrogen blistering because it introduces hydrogen to the surface of the steel
◉ Examples of Cathodic Poisons: Answer: - As, P, Sb, Bi, S, Se, Te, CN-, H2S
- addition of arsenic trioxide to acid, the presence of the As slows the cathodic reaction therefore slowing corrosion ◉ Why are cathodic poisons bad? Answer: they are very harmful to the environment and people ◉ How do Scavengers work? Answer: - work by eliminating oxygen from a closed system with a neutral or alkaline pH
- with no oxygen for a cathodic reaction the corrosion is slowed
- boiler waters are often vacuum degassed to remove most of the oxygen, then a scavenger is introduced to tie up the remaining O ◉ Example of how Scavengers work: Answer: water flooding - inject water to build up pressure to increase oil recovery ◉ Why are Scavengers not as effective as acids? Answer: - Scavengers remove O2, not H+ ions, therefore are not as effective as acids
◉ Types of Coatings: Answer: 1. Metallic coatings
- Conversion coatings
- Organic coatings ◉ Metallic coating can be either... Answer: - anodic or cathodic to the substrate metal
- if anodic it will act in a sacrificial manner
- if cathodic it must provide a complete barrier, any pores or scratches could cause a galvanic cell which can lead to rapid attack of substrate that are exposed to environment ◉ What is critical to any type of coating system? Answer: proper surface preparation prior to applying the coating ◉ Ways to clean the surface before coating: Answer: 1. Degreasing (surface must be clean)
- Mechanical preparation (abrading)
- Pickling (chemical preparation, remove oxide film and lightly etches the surface)
◉ Methods used to apply coatings: Answer: 1. Electroplating
- Hot dip coatings
- Vapor deposition
- Thermal spray coatings
- Cladding 6 Diffusion coatings ◉ Materials that can be electroplated: Answer: - nickel
- electroless Ni plating
- chromium
- zinc
- cadmium
- tin ◉ Materials that can be used for hot dip coatings: Answer: - zinc galvanizing
- aluminum
- 55Al-Zn ◉ How does zinc galvanizing hot dip coating work?
◉ What are advantages and disadvantages of vapor deposition? Answer: - complex shapes can be uniformly plated
- very dense and very adherent coatings
- difficult to achieve thick coatings and tends to be expensive ◉ How does thermal spray coatings work? Answer: - coating material is fed through a gun where it is melted and blown by a stream of compressed gas onto the substrate
- as the droplets strike the surface they flatten into platelets that adhere and conform to the surface ◉ What are advantages of thermal spray coatings? Answer: - can apply thicker coatings than hot dip coatings
- can apply alloys ◉ What different materials can be used for thermal spray coatings? Answer: - aluminum and zinc are commonly used for corrosion protection
- tungsten carbide or chromium carbide coatings are used for wear resistance ◉ How does cladding work?
Answer: - thin sheet of the cladding metal is bonded to the base metal by rolling or hot pressing
- this can be done by "alcladding" which is when a high strength aluminum alloy is coated with a thin layer of pure aluminum or a corrosion resistant aluminum alloy ◉ How do diffusion coatings work? Answer: - the part to be treated is packed in Al, Zn, Cr, or Ti and is heated
- the packing metal diffuses into the substrate form intermetallics at the surface of the part
- these intermetallics may aid in limiting corrosion losses ◉ How do conversion coatings work? Answer: these coatings convert the surface of the metal to an inorganic compound that is corrosion resistant and extremely adherent ◉ Examples of conversion coatings: Answer: - Anodizing
- Phosphate coatings
- Oxide coatings ◉ When is anodizing typically used?
- water resistant
- proper adhesion
- abrasion resistance
- aging and UV resistant
- ability to expand and contract with the metal it is protecting ◉ What do most coating failures results from? Answer: - poor surface preparation prior to appyling the coating
- surface prep may be the most expensive part of the job cost, but it is critical to the longevity of the coating ◉ What is the second most important reason for coating failures? Answer: - improper application
- must follow all directions ◉ What are the direction for coatings that must be followed? Answer: - mixing coating (proper proportions and wait times)
- method of application (spray, brush, roller)
- paint mil thickness
- coating must be evenly applied and cover all areas
- must be applied to acceptable conditions (humidity, temperature)
- allow sufficient cure time before recoating or putting into service
- follow the coating system (proper primer and number of coats) ◉ Various types of organic coatings: Answer: 1. Inert coatings
- Sacrificial coatings: zinc rich
- Corrosion inhibiting coatings: contain pigments which act to absorb moisture that penetrates the paint film ◉ Examples of coating pigments: Answer: - Zinc phosphate
- Zinc chromate
- Barium metaboride
- Strontium chromate ◉ Most pain coatings consist of 5 main components: Answer: 1. Resin (or binder)
- Pigments
- Extenders
- Solvent
- Special additives (surfactants, dryers, antifungicides) ◉ What does the cathodic disbondment test evaluate?
Answer: the metal that is to be protected can be forced into passivating and the corrosion rate should drop off significantly ◉ Why is anodic protection not commonly practiced? Answer: because any breakdown in the system could result in rapid corrosion and subsequent failure ◉ What are the parts needed in anodic protection? Answer: - reference electrode
- rectifier
- potentiostat ◉ What does the potentiostat do? Answer: maintains the anodes potential at the required value ◉ How does the process of anodic protection work? Answer: - the desired potential for passivation is set on the potentiostat
- this in turn compares this potential with the potential of the tank as measured versus the reference electrode
- if the two potentials differ, the current between the tank and the cathode (inside tank) is automatically adjusted by the potentiostat until potentials are the same
◉ Why is localized corrosion considered very dangerous? Answer: it is often harder to detect and if undetected can proceed until a sudden failure occurs ◉ Main forms of localized corrosion: Answer: - galvanic corrosion
- crevice corrosion
- pitting corrosion
- erosion corrosion
- intergranular corrosion
- dealloying
- environmentally induced cracking (SCC and HIC)
- microbially induced corrosion (MIC) ◉ What is another name for galvanic corrosion: Answer: bi-metal corrosion or two metal corrosion ◉ What are the requirements for galvanic corrosion to occur? Answer: - electrochemically dissimilar metals must be present
- these metals must be in electrical contact
- these metals must be exposed to a common electrolyte
