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Correct CHOICE is (a) FERROUS ions

Best explanation: A under film corrosion filament consists of an active head and INACTIVE tail. The active head CONTAINS ferrous ions, that have a characteristic COLOR of blue-green. Whereas inactive tail contains ferric oxide and hydrated ferric oxide ions, that has a characteristic color of red-brown.

Correct option is (d) Metal cladding, electroplating and metal coat extrusion

The explanation: Corrosion is an electrochemical reaction that RESULTS in the degradation of a MATERIAL. It can be PREVENTED by VARIOUS metal coating methods such as metal cladding, electroplating and metal coat extrusion.

Correct choice is (d) Zinc dissolution in dilute H2SO4 and rusting of iron

Explanation: Uniform corrosion is characterized by an electrochemical REACTION that proceeds uniformly over the ENTIRE EXPOSED surface. Zinc dissolution in dilute H2SO4 and rusting of iron are examples of uniform corrosion. Dezincification of brass in ACIDS is an EXAMPLE of selective leaching.

The CORRECT option is (a) True

Easiest explanation: Selective LEACHING is one of the types of corrosion that results in the REMOVAL of one element from a solid ALLOY by the corrosion process. Dezincification, dealuminumification, and decobaltification are EXAMPLES of selective leaching.

Right answer is (a) Vertically downwards

The best explanation: Pits usually grow in the direction of gravity i.e. vertically downwards. Gravitational force HOLDS the corrosive solution vertically downwards in a cavity, that makes the pit GROWTH downwards. And very a smaller number of pits GROWN vertically upwards.

The correct ANSWER is (a) True

The explanation: CREVICE CORROSION index (CCI) is developed by Streicher. CCI is an index used to assist the SELECTION of materials for construction. It minimizes the chances of crevice formation by PROPER material selection.

Correct choice is (d) A narrow active-PASSIVE TRANSITION, a small critical current density, and an extended passive REGION

The explanation: The optimum crevice corrosion resistance of active-passive transition metal is acquired by having the following CHARACTERISTICS: i. A narrow active-passive transition

ii. A small critical current density

iii. An extended passive region

The correct choice is (d) The stagnant solution in the holes, GASKET SURFACES, lap joints, and SURFACE deposits

Explanation: Crevice corrosion is a TYPE of corrosion in which dissolution of metal is due to the presence of stagnant solution in the small gaps (crevice) such as holes, gasket surfaces, lap joints, and surface deposits, ETC.

The correct choice is (c) Dealloying and PARTING

The explanation: Selective Leaching is the REMOVAL of one element from a solid ALLOY by the corrosion process. The other NAMES of selective leaching are dealloying and parting.

Correct choice is (d) NIOBIUM, columbium, and titanium

The BEST I can explain: Stabilizing ELEMENTS are the elements that form strong carbides than chromium. These elements have a much greater AFFINITY to carbon than does chromium, this minimizes the chromium depleted zone.

Right ANSWER is (d) ACETIC acid, acetic acid+salicylic acid, ammonium nitrate

The explanation: STAINLESS steel is highly susceptible to intergranular CORROSION. These corrosives such as acetic acid, acetic acid+salicylic acid, ammonium nitrate, etc. Induces the intergranular corrosion in austenitic stainless steel.

The correct answer is (b) Leaflike structure

Easiest explanation: Mahla and NIELSEN of Du PONT, using the electron microscope, have shown that the carbides form on the envelope around the GRAINS in a leaflike structure.

Right option is (B) GRAIN boundary

Explanation: Intergranular corrosion OCCURS when grain interfaces or grain BOUNDARIES are very reactive than the face of grain. Whereas in the UNIFORM attack, grains are more reactive than the grain boundaries.

The CORRECT CHOICE is (a) Monel and copper

For EXPLANATION: The galvanic effect can be reduced by selecting close metal COMBINATIONS. The position of a metal or an alloy in galvanic is MAINLY affected by its composition and its state (active or passive). Monel (70 Ni, 30Cu) and copper is the close metal combination, hence it has a minimum galvanic effect.

Correct ANSWER is (a) PRINCIPLE of GALVANIC corrosion

Best explanation: Cathodic protection is a protection of a base STRUCTURE metal by making it as a cathode. It can be protected by the use of SACRIFICIAL anode. The order of metals in the galvanic series is the basis for cathodic protection.

The CORRECT choice is (a) Grey cast IRON

Explanation: Graphitization occurs only in grey cast iron whereas it doesn’t occur in nodular, MALLEABLE, and white cast iron as the graphite network is not PRESENT to hold together at the surface.

Right option is (a) True

The explanation is: Carbon pickup or surface CARBURIZATION during the production of austenitic STAINLESS steel has caused premature failures. It occurs when these steels are cast into molds that contain carbonaceous materials such as ORGANIC BINDERS and baked oil sand. It reduces the corrosive resistance of these steels in the viewpoint of intergranular ATTACK.

Correct CHOICE is (d) The stagnant electrolytic solution should be present

Easiest explanation: Crevice corrosion occurs due to the PRESENCE of stagnant electrolytic solution in the crevices. It acts at the corrosion sites and it results in the formation of LOCALIZED corrosion. An increase in the velocity of the electrolyte DECREASES the extent of crevice corrosion.

Right option is (d) IRON in aluminum, zinc in brass and 18-8 stainless STEEL

Best explanation: All the given alloys are prone to intergranular corrosion. Iron in aluminum and zinc in brass are SUSCEPTIBLE to this corrosion, due to the segregation of iron and zinc respectively at the grain boundaries. Whereas 18-8 Stainless steel is susceptible due to the depletion of CHROMIUM at the grain boundaries.

Right choice is (d) It should be wide enough to permit liquid entry and narrow enough to maintain a stagnant ZONE

For explanation I would say: Crevice is a small volume region formed between the interface of METAL and gaskets. It acts as a corrosion SITE if is wide enough to permit liquid entry and narrow enough to maintain a stagnant zone.

The correct option is (d) LIMITED to pure metals and measured only at standard CONDITIONS

The explanation: EMF series is a LIST of pure metals arranged according to their electrode POTENTIAL value measured against SHE. It is measured at standard conditions such as 25°C, 1atm pressure, and 1M concentration of hydrogen ions.

Correct OPTION is (c) CATHODIC area/ANODIC area

To explain I would say: It is the ratio of cathodic to anodic areas. An unfavorable area ratio with a large cathodic area and SMALL anodic area results in high current density at the ANODE. The greater the current density at an anodic area the greater the corrosion rate.

The correct option is (d) Hydroxide, chromate and silicate SALTS

To EXPLAIN I would SAY: PITTING is one of the most destructive and insidious forms of corrosion. The tendency of pitting can be increased with the presence of chloride, bromide, and hypochlorites. But this tendency can be prevented or minimized with the presence of hydroxide, chromate, and silicate salts.

The correct option is (d) 6 months – 1 year

Easiest explanation: The incubation PERIOD is the time REQUIRED to initiate the CREVICE corrosion on METAL. It usually in the range of 6 months to 1 year. It depends on many factors such as the material used, corrosive medium, temperature, ETC.

Right option is (B) -2.925 V

For explanation: STANDARD REDUCTION potential values of metal are measured against SHE, which is arbitrarily defined as ZERO. Standard reduction potential of potassium (K-K^+), ALUMINUM (Al-Al^+3), zinc (Zn-Zn^+2), copper (Cu-Cu^+2) are –2.925V, -1.662V, -0.763V and +0.337V respectively.

The CORRECT CHOICE is (a) Appearance of corroded metal

The explanation is: The BASIS for the CLASSIFICATION of CORROSION is the appearance of corroded metal. Each form can be identified by visual observation as it has a unique form of appearance.

Correct option is (d) 1950°F-2050°F

The BEST I can explain: COMMERICAL solution-quenching treatment consists of HEATING to 1950°F to 2050°F followed by water quenching. Chromium carbide is dissolved at these temperatures, and a more homogenous alloy is formed. It minimizes the susceptibility of WELD metal to weld DECAY.

Correct choice is (c) Initiation method

The best I can explain: Pitting is a special CASE of CREVICE CORROSION. But in the view of the initiation method, these two-corrosion processes are DIFFERENT. Pitting corrosion is a self-initiating corrosion FORM, whereas crevice corrosion needs the formation of crevice (a small gap) with differential concentration from outside.

Right answer is (a) True

For EXPLANATION I would say: As galvanic CORROSION is a bi-metallic corrosion, RELATIVELY less corrosion-resistant metal preferentially acts as ANODE and relatively HIGH corrosion-resistant metal preferentially acts as a cathode. This increases attack on the less corrosion-resistant metal (anode) and decreases on high corrosion-resistant metal (cathode).

The correct choice is (c) Heating the welded STABILIZED STEEL to 1950°F

The best I can explain: The temperature RANGE of 950°F to 1400°F is the sensitizing zone where only chromium carbide precipitates. Heating the welded stabilized steel to 1950^0F results in the dissolution of chromium carbide and precipitation of columbium carbide.

The correct answer is (d) Enrichment of silicon on stainless steel for better passivity, preparation of Raney NICKEL catalyst and EXTRACTION of metals using hydrometallurgy

The explanation is: The benefits ASSOCIATED with SELECTIVE leaching are

i. Enrichment of silicon on Stainless steel for better passivity

ii. Preparation of Raney nickel catalyst

iii. Extraction of metals using hydrometallurgy

The correct answer is (a) True

The EXPLANATION is: Welding time and the peak temperature of welding are the two parameters of WELD decay. Electric ARC welding produces more INTENSE heat in a shorter time whereas in gas welding it is LESS intense and requires more time for welding.

The correct option is (c) CHROMIUM depleted zone NEAR the grain boundary

Explanation: !8-8 stainless steel is SUSCEPTIBLE to intergranular corrosion due to sensitization in the temperature of 950 to 1450°F. At this stage, chromium reacts with CARBON near the grain boundary and RESULTS in the formation of the chromium depleted zone. This chromium depleted zone is prone to intergranular corrosion.

The correct option is (d) LOCALIZED attack at and adjacent to the grain boundaries

The best explanation: Intergranular CORROSION MEANS corrosion in between two or more GRAINS. Intergranular corrosion is a localized attack at and adjacent to the grain boundaries.

Right option is (b) Cavity with the surface DIAMETER about the same as or less than the depth

The best I can EXPLAIN: Pitting is extremely localized corrosion that RESULTS in pits or cavity. A pit is a cavity with the surface diameter about the same as or less than the depth.

Correct OPTION is (c) AFFECTS surface appearance

Best explanation: This FILIFORM corrosion affects only surface appearance on metal-coated food and BEVERAGE cans that are exposed to the high humid atmosphere. It doesn’t weaken the metal as well as decreases the tensile strength.

Correct ANSWER is (d) Tantalum

The best I can explain: Tantalum is the most corrosion-resistant metal at room temperature. It is a rare, hard and lustrous TRANSITION metal. It is denoted by Ta and its atomic NUMBER (Z) is 73.

Correct option is (d) EROSION, galvanic and PITTING corrosion

Explanation: Types of corrosion: i. UNIFORM corrosion

ii. Galvanic corrosion

iii. Crevice corrosion

iv. Pitting corrosion

v. INTERGRANULAR corrosion

vi. Selective leaching

vii. Erosion corrosion

viii. Stress corrosion

Right choice is (d) INCOMPLETE weld penetration, EXTREMELY POROUS casting and improper heat treatment

The best I can EXPLAIN: Corrosion is an electrochemical reaction that results in the degradation of a material. Corrosion failures are usually catastrophic in nature. And the CAUSES are incomplete weld penetration, extremely porous casting, and improper heat treatment.

The correct option is (a) PITTING and crevice corrosion

The best I can explain: Autocatalytic is a phenomenon of rapid corrosion that occurs within the pit or crevice, while oxygen reduction TAKES PLACE on adjacent surfaces. This process is self-stimulating and self-propagating. It occurs in both pitting and crevice corrosion.

The correct answer is (b) Pits cathodically PROTECT adjacent surfaces

To EXPLAIN: Pits cathodically protect adjacent surfaces of metal; this is also called as CATHODIC protection. This is due to the AUTOCATALYTIC nature of PITTING corrosion. Hence, pits cathodically protect adjacent surfaces is the correct statement.

The correct answer is (B) PITTING CORROSION

To explain: Case A: No corrosion of metal

Case B: Intense pitting on specific points

Case C: UNIFORM corrosion

The correct ANSWER is (b) True

Best explanation: Pitting can occur in TWO steps: i. PIT INITIATION ii. Pit growth.

The incubation period is the time required for a system to initiate pits on the metal surface. It usually ranges from months to years.

Correct ANSWER is (c) PITTING corrosion

Explanation: Pitting is ONE of the most DESTRUCTIVE and insidious FORMS of corrosion. It causes equipment to fail because of perforation with only a small percent weight loss of the entire structure.

Right answer is (b) Canning industry

The explanation: The Canning industry has a MAJOR CONCERN on filiform corrosion. As canning is a preservation method by sealing the container’s airtightly and APPLYING EPOXY coating on the metal SURFACE. Whereas the petroleum and automobile industry face uniform and galvanic corrosion as a major concern.

Correct OPTION is (a) Relative humidity of the atmosphere

The explanation is: Relative humidity of the surroundings is the most environmental FACTOR of filiform corrosion. It occurs primarily between 65% to 90% relative humidity. If the relative humidity is less than 65%, the METAL is UNAFFECTED.