Basic Corrosion Flashcards
What is the definition of corrosion?
Deterioration of material or properties because of reaction with its environment
Which of the following is an indirect cost of corrosion?
Loss of redundancy
Which of the following is a direct cost of corrosion?
Structural collapse
Product contamination
In an electrochemical corrosion reaction, which of the following statements applies to the electrons?
They flow through the metal
Reduction occurs when which of the following happens?
Negative charge increases
Which of the following is involved in an anodic reaction?
Loss of electrons
In which of the following situations would electrochemical oxidation most likely occur?
Electrons are lost
Which of the following metals does not normally corrode in natural environments?
Gold
Which of the following relates to the Nernst Equation?
Potential of pure metals in solutions containing various concentrations of ions
Which of the following relates to the Electromotive Force (EMF) series?
Lists metals in order of their potential in solutions of standard ion concentrations
What does the term “polarization” describe?
Changes in potential due to passage of electrical current
When a metal cannot oxidize or corrode, it is said to be which of the following?
Passive
A pH greater than 7 indicates that a substance is which of the following?
A base
Which of the following applies to hot water when oxygen is present?
It is more corrosive than cold water
Hygroscopic materials absorb which of the following?
Water
Which of the following is generally true of saltwater when oxygen is present?
It is more corrosive than freshwater
A bare steel piling in saltwater experiences the highest corrosion rate in which location?
Splash zone
The submerged portion of a bridge piling over a marine waterway is an example of an item that must be able to resist corrosion from which of the following?
Salts
Which of the following pH values would result in the lowest corrosion rates for steel?
11.0
Generally, what is the effect of increasing temperature on corrosion?
Increased corrosion reactions
Which of the following descriptions applies to a cast structure?
Achieved by solidification from molten metal
Which of the following descriptions applies to wrought structures?
Altered by thermo-mechanical process
Which type of stainless steel is commonly used for scissors and surgical instruments?
Martensitic
The majority of stainless steels used worldwide are which type?
Austenitic
Which alloy is added to steel to sharply decrease oxidation at elevated temperatures?
Chromium
What are the preferred casting alloys for large pumps and valves in seawater?
Nickel aluminum bronze
Which of the following applies to aluminum in marine atmospheres?
It does not usually need to be coated
As thermoplastic polymers are heated, what generally occurs?
They soften
Silicones are generally considered less resistant to heat and chemical attack than which of the following?
Nickel alloy
What is corrosion that occurs uniformly over the surface of a metal or alloy called?
General
Which type of corrosion occurs in shielded areas where free access to the bulk environment in surrounding areas is restricted?
Crevice corrosion
Which of the following are common ways to control crevice corrosion?
Sealing lap joints where they cannot be avoided
Providing complete drainage
What is the formation of subsurface planar cavities in a metal from excessive internal hydrogen pressure?
Hydrogen blistering
Which type of corrosion occurs due to electrical contact between two dissimilar metals in an electrolyte?
Galvanic
Tensile stress and corrosion in the presence of water and H2S can cause which of the following?
Sulfide stress cracking
Which form of corrosion is characterized by the premature failure of cyclically loaded part?
Corrosion fatigue
Which of the following is an attack resulting from the combined action of corrosion and fluid flow on stationary or moving surfaces?
Erosion-corrosion
Which of the following descriptions applies to dealloying?
Selective removal of one element from an alloy by corrosion processes
What type of corrosion occurs due to repeated relative motion between two surfaces under load in the presence of a corrosion environment?
Fretting
What does the galvanic table contain?
Galvanic series
Using the galvanic table, what metals will be the least corrisive together and why?
Copper and stainless steel aloys because the closer they are in the series the more compatible.
What does the galvanic table indicate?
The need or degree of protection to be applied to reduce expected corrosion in a galvanic and salt water environment.
How can the corrosion of magnesium aloys be diminished?
By the use of the proper protective finish.
What does the formation of a tightly adhering oxide film on aluminum surfaces produce under mild corrosive conditions?
Increased conditions.
What type of corrosion indicator is usually found on steel?
Red rust.
What type of corrosion indicator is usually found on copper?
Blue, or blue green.
What type of corrosion is most commonly found on copper?
Interangular corrosion.
Why are stainless steels more resistant to common rusting?
Due to passive films.
How does stainless steel interact with almost any other metal if proper techniques of sealing and protective coating are ignored?
They can cause galvanic corrosion.
What type of insulation is required between titanium and other metals?
Electrical insulation.
How are nickel and chromium used and what makes them so uniquely useful?
Used as protective platings, they form continuous oxide coatings protecting themselves and underlying metals.
Which of the following defines high-temperature corrosion?
Phenomenon that requires a specific environment and applied stress.
Formation of thick oxide or sulfide scales that cause internal swelling of the metal.
Which of the following is the most common alloy used for high-temperature exposure?
Stainless steel.
What is the primary purpose of post-weld heat treatment?
To reduce residual stresses from welding.
In design, which of the following chemical factors that can influence corrosion should be considered?
Major species
Minor species
Nature of environment
Which of the following must the design engineer take into consideration for upset temperature conditions?
Maximum operating temperature.
In designing against dissimilar metal (galvanic) corrosion, what is often more important than the position of the metals in the galvanic series?
Cathode/ anode area ratio.
Good design to minimize corrosion will ensure which of the following?
Water or corrodents cannot be trapped.
Which of the following is important in good design for inspection and maintenance?
Easy access.
If good design practices are being utilized, how can crevices at joints be avoided?
Full penetration welding instead of bolting or riveting.
Corrosion allowance is used in design for environments where the metal corrodes in which of the following ways?
Uniformly at a linear or decreasing rate with time.
During design, when must the environment be defined?
Before appropriate materials are selected.
Where safety is critical, such as in transportation of hazardous products, what is more important than cost?
Reliability.
Which of the following are important corrosion-related factors to be considered in the selection of metallic materials?
Mechanical properties of materials
Cost of materials
Which of the following are common methods for corrosion control?
Corrosion inhibitors
Materials selection
Coatings
Anodic corrosion inhibitors must cover the entire anodic area of metal surface to avoid which of the following?
Accelerated localized corrosion of anodic areas.
What is the purpose of using a biocide, such as chlorination, in a water treatment?
To control microbiological fouling.
Which of the following are common design defects that make coating application difficult?
Riveted construction
Threaded areas
Weld spatter
Which of the following is a common galvanic anode material for protection of steel?
Zinc
Which of the following are examples of impressed current anodes?
Silicon cast-iron and graphite.
In an impressed current system, the positive terminal of the power supply must always be connected to which of the following?
Anode groundbed
What is one of the simplest and least expensive non-destructive inspection methods?
Visual examination.
Which type of inspection method can non-destructive gauge the thickness of in-service equipment?
Ultrasonic.
Which inspection method can locate crack-like surface defects on a variety of non-porous materials?
Dye penetrant.
Which inspection method can detect defects voids, inclusions and pit depths but is less effective in locating cracks unless the orientation of the cracks is known?
Radiography.
What is the most commonly used method for corrosion monitoring?
Weight-loss coupons.
In galvanic corrosion monitoring, accurate measurements of current flow between dissimilar metal electrodes is achieved with which of the following instruments?
Zero-resistance ammeter.
In water chemistry monitoring, what can metal-ion concentration analysis indicate?
Changes in the corrosivity of the environment.
What is the most common method for monitoring the effectiveness of cathodic protection of a structure?
Measuring structure-to-electrolyte potential.
What is the benefit of eddy current inspection?
It can be used on electrically conductive material.
What are identifying characteristics of corrosion on silver or gold?
Silver is a brown black film, and gold is a darkening on reflective surfaces.
Where is carbon in the galvanic series?
Composites.
Why is separation of carbon/graphite metals to other metals so important?
To prevent dissimilar metal attack on the attached part.
What are the four basic standard stages of corrosion prevention and control?
Cleaning, protection, inspection, control.
What type of cleaner is used when cleaning metals in a corrosive environment and how
often should they be cleaned?
With the mildest soap possible and frequently.
What should paint surfaces be inspected for?
Itregularities, blisters, flakes, chips.
List the types of procedures used when implementing corrected treatment when corrosion is detected:
Thoroughly inspect all corroded areas, evaluate damage, remove paint/corrosion, apply chemical surface treatment, sealing, and apply paint finishes.
What are the more common types of finishes used on board naval vessels?
Paint, powder coating, and chemical conversion coating.
How are exposed metals treated?
They are wiped down regularly with oil.
What is the most common example of
corrosion?
Rust on steal.
What four conditions must be present for electrochemical corrosion to occur?
Anodic area, cathodic area, metal, electrolites.
What is the best way to stop the electrochemical corrosion process?
Remove any of the four conditions.
What are some of the environmental conditions that will cause corrosion?
Temperature, moisture, salt, atmospheres, ozone, sand, dust, climate, animals, bacteria, volcanic ash, microorganisms.
What is the substance that may contain contaminants such as chlorides, sulfates, and nitrates?
Man made.
What is the temp for mold and bacteria to grow on metals?
104° F
Salt particles disolved in water form what?
A strong electrolyte.
How much salt is carried on a cubic mile of air in a normal sea wind?
10 to 100 lbs.
What two conditions when combined with ozone have to be present to increase the rate of corrosion?
Electrolyte solutions in contact with metals.
What substances may absorb and hold moisture and be sufficient to promote corrosion?
Sand, dust, volcanic ash.
How do animals, microorganism, and bacteria cause corrosion?
By introducing moisture, secreting corrosive fluids, depleting oxygen or oxidizing organic compounds such as iron, sulfar, hydrogen, carbon monoxide.
What types of industrial pollution cause corrosion?
Carbon, Nitrates, Ozone, Sulfar dioxide,
Sulfates, such as exhausts from engines.
List the severity factors that determine how sever corrosion will be:
Electrolytes, temperature, metal, time, mechanical stress, biological organisms.
What are the common types of corrosion?
Galvanic, uniform, intergranular and filform, stress, oxidization, pitting, crevice.
What are the characteristics of uniform
corrosion?
Consistant cover of corrosion over a large surface, rough surface, frosted appearance.
What type of corrosion is galvanic corrosion?
Electromechanical.
What components have to be present for galvanic corrosion and how is it recognized?
2 dissimilar metals placed in contact joined by an electrolyte, recognized by built up corrosion at the joint.
What are the differences between intergranular and exfoliation?
Exfoliation is an advance form of intergranular, and has swelling and lifting from corrosion.
How does a technician recognize pitting?
When the deposit is cleans away tiny pits are visible.
When does a crevice/concentration cell corrosion occur?
When an electrolyte has different levels of concentration.
How does the concentration make up of electrolyte affect metal surfaces in crevice/ concentration cell corrosion?
Corrosion to occur inside a crevice.
What type of corrosion occurs when a foreign material covers a surface?
Crevice concentration cell corrosion.
Explain the relationship of metal ions in corrosive electrolytes between two joined metal surfaces?
Will normally have a high concentration of metal ion.
How do the metal ions create a cathodic/anodic corrosive affect on joined metal surfaces?
The area of the metal in contact with the higher concentration of metal ions will be cathodic and show no signs of corrosion, the lower area will be anodic and show
corrosion.
How does oxygen create a corrosive affect?
Oxygen cells develop under gasket, wood, rubber, plastic in contact with metal surfaces, and electrolytes.
What type of alloy most often displays oxygen concentration cell corrosion?
Stainless steel.
List the steps that active/passive corrosion process takes:
Deposit of dirt or salt that creates an oxygen concentration cell broken in the area of the salt deposit.
What sort of corrosion occurs during
active/passive concentration?
Pitting.
What type of corrosion is filiform? How is it recognized?
Crevice corrosion.
What factors determine the type of corrosion that occurs?
Type of metal, environment, presence of protecting coatings.
On the galvanic table how are metals arranged?
In order of their relative activity in a sea water environment.
What is the process of iron corrosion?
A chemical change and an electrical process.
What is an example of a chemical change?
Turning iron into iron oxide.
What are three ingredients for a corrosion battery?
Oxygen, Water, Salt (Ions)
Smog, salt spray, marine locales, industrial soot, splash and spillage are examples of:
Ions.
Where does rust form during a corrosion battery?
Anode.
Where oxygen turns into caustic ion, is not attacked by rust:
Cathode.
Where edges and other areas of mechanical work (drilled, sheared, etc.) are prone to anodes:
Steel corrosion.
Does not form rust on anode, cathode makes hydrogen ion into hydrogen gas bubbles:
Acid corrosion battery.
Two metals connected electrical, one corrodes faster and preferentially than other:
Galvanic corrosion.
System of ranking tendency of corrosion, higher on scale more likely to corrode and protect metal lower on scale:
Galvanic Scale.
Anode metal area smaller than cathode metal area, copper plates and steel rivets:
Severe Galvanic Corrosion.
Anode metal area larger than cathode metal area, steel plates and copper rivets:
Mild Galvanic Corrosion.
Oxygen starved area becomes anode, outside area becomes cathode, coating over area not good:
Crevice corrosion.
Protective film breaks down, anode formed on unprotected area, surrounding metal is cathode, smaller the area the worse the rust:
Pitting corrosion.
Iron oxide layer on steel, is brittle and cracks easily, is cathodic and makes steel anodic causing major rust on unprotected areas:
Corrosion under mil scale.
Galvanic, crevice, pitting, corrosion under mil scale:
Types of corrosion.
Highest on galvanic scale:
Magnesium.
Lowest on galvanic scale:
Platinum.
Altering the environment, corrosion resistant materials, cathodic protection, overdesign, barriers:
Methods of corrosion control.
Controlling accidental discharge of corrosion vapors, low cost, limited to immersion solution:
Altering the environment.
Positively or negatively charged atom or group of atoms is-
Atom? Electrolyte? Electron? Ion?
lon
Smallest unit of an element is-
Atom? Electrolyte? Electron? Ion?
Atom
Liquid solution, usually water, containing ions is-
Atom? Electrolyte? Electron? Ion?
Electrolyte
Negatively charged particle much smaller than an atom is-
Atom? Electrolyte? Electron? Ion?
Electron
Which of these is categorized as a coating failure?
Cratering
Cracking
Alligatoring
Chloride contaminations of steel substrates is known to cause:
Osmotic blisters.
An inhibitor is a:
Substance that retards a chemical reaction.
Define Corrosion
The deterioration of a substance, usually a metal from a reaction with its environment.
What is Passivation?
A layer of oxides formed on the surface of a metal that provides corrosion protection. Stainless Steel for example.
What are the elements of a corrosion cell?
Anode
Cathode
Metallic Pathway
Electrolyte
Describe what happens at the anode:
The metal dissolves into the electrolyte.
What is the function of the Electrolyte?
Allows the passage of ions to connect the corrosion cell.
What is the function of the Metallic
Pathway?
Allows the flow of electrons to connect the corrosion cell.
What is the galvanic series?
A listing of metals in order of reactivity (more or less noble) in seawater at 25 degrees C.
The general rules of galvanic corrosion are:
The less noble (or more reactive) metal when connected to a more noble (or less reactive) metal will corrode preferentially.
Name the 5 most important factors that affect the rate of corrosion:
Humidity
Oxygen
Chemical Salts
Pollutants
Temperature
General Corrosion is:
Results in a relatively uniform loss of material over the entire surface.
Results in a general thinning of the affected surface.
Relatively easy to inspect.
Does not cause catastrophic failures.
Localized Corrosion is:
Typically Pitting and Crevice Corrosion. It is typically of the most concern as damage is concentrated and loss of integrity or structural failure can result.
List some of the common tools/methods used in corrosion control:
Designs
Inhibitors
Material Selection
Cathodic Protection
Protective Coatings
Splash Zone Systems
Alteration of the Environment
Oxidizing anodic inhibitors require the presence of oxygen. True or false?
False
Silicates and phosphates are common oxygen scavengers. True or false?
False
Inhibitors may be added to coatings. True or false?
True
Cathodic protection can control corrosion. True or false?
True
Concrete is generally resistant to acidic environments. True or false?
False
Corrosion control at the design stage must be planned. True or false?
True
A corrosion inhibitor is a substance which, when added to an environment, decreases the rate of attack by the environment. True or false?
True
Passivating inhibitors cause a shift in cathode potential, causing the cathode to become more positive. True or false?
False
Ohmic inhibitors increase resistance of electrolyte circuit by formation of a film on the metal surface. True or false?
True
Ohmic inhibitors may affect the entire surface of the metal. True or false?
True
Approximate cathodic protection current requirements for bare steel in moving seawater are:
100-160 mA/M2
A cathodically protected structure may be coated primarily to:
Reduce the area exposed to the electrolyte.
Sacrificial coatings:
Corrode instead of a steel substrate.
Act as a barrier between the substrate and the corrosive medium.
Corrosion detection is used for:
Safety
Cost effectiveness
When a quantity of inhibitor is periodically added to a closed system, the technique is in use:
Batch treatment
In seawater, which of the following metals is anodic with respect to steel?
Zinc
Stress corrosion cracking is usually accompanied by significant metal loss. True or false?
False
Stress corrosion cracking is a cathodic phenomenon. True or false?
False
Hydrogen induced cracking may occur at cathodes. True or false?
True
Hydrogen induced cracking occurs when molecular hydrogen evolves as a gaseous cathodic reaction product without penetrating the metal substrate. True or false?
False
Conversion of atomic hydrogen to molecular hydrogen may be inhibited by arsenic compounds. True or false?
True
Sulfide stress cracking may be encountered where wet H2S is present. True or false?
True
Liquid metal embrittlement may occur if a metal is stressed in tension while in contact with an active liquid metal. True or false?
True
Cathodic protection may contribute to hydrogen damage in metals. True or false?
True
Cathodic protection may be helpful in controlling cavitation corrosion. True or false?
True
Crevice corrosion occurs:
Occurs most commonly on film-protected metals.
Occurs where free access to environment is restricted.
Filiform corrosion may occur:
Beneath coatings
Electrode potentials are sensitive to:
Temperature
Presence of depolarizers
In a galvanic couple, which of the following is likely to promote the most rapid corrosion reaction?
Large cathode area, small anode area
Nearly all metals exhibit crystalline structures. True or false?
True
Thermoplastic materials become and remain pliable whenever heated. True or false?
True
Thermoset materials harden and remain hard after one exposure to heat. True or false?
True
Plastics are generally not prone to “cold flow” or “creep.” True or false?
False
Metals conduct electricity through the flow of ions. True or false?
False
Plastics are subiect to corrosion by oxidation. True or false?
True
Plastics may deteriorate due to continued polymerization. True or false?
True
Concrete may corrode. True or false?
True
Reinforcing steel in concrete does not present a corrosion risk. True or false?
False
Water used to make concrete should be free of chemical contaminants. True or false?
True
Corrosion rates generally increase when temperature increases. True or false?
True
_____________ tend to leach calcium trom concrete.
Soft waters
____________ is a chemical substance which, when added to an environment, decreases the rate of attack by that environment
Corrosion inhibitor
Pourbaix diagrams predict the potential of metals in environments with varying pH.
False
Passivity of a metal can be enhanced by alloying it with a more active metal. True or false?
True
Martensitic alloys contain:
12-17% chromium
Polluted air is more corrosive than clean air. True or false?
True
Salt water is more corrosive than fresh water. True or false?
True
Corrosion is the deterioration of a substance (usually a metal) or its properties because of a reaction with its environment. True or false?
True
Electrons are negatively charged. True or false?
True
An ion is a neutral atom. True or false?
False
The cathode is an electrode where oxidation reactions occur. True or false?
False
Passivity is due to formation of a surface film which acts as a barrier to further corrosion. True or false?
True
Metallic ions may plate out on cathodic surfaces (as opposed to anodic surfaces). True or false?
True
Moisture promotes corrosion by acting as an electrolyte. True or false?
True
Corrosion rates generally increase when temperature increases. True or false?
True
Electrochemical reactions can occur without the transfer of electrons. True or false?
False
Electrons are said to be:
Negatively charged particles
____________ allows electronic current to flow from anode to cathode.
Metallic path
Factors influencing corrosion in liquid environments include:
Chemistry of system
Flow rate of liquid
Temperature of liquid
