250+ TOP MCQs on Mineral Acids – Hydrofluoric Acid and Phosphoric Acid and Answers

Corrosion Engineering Multiple Choice Questions on “Mineral Acids – Hydrofluoric Acid and Phosphoric Acid”.

1. Which of the following material is/are primarily used for hydrofluoric acid?
a) High-silicon cast iron
b) Magnesium
c) Aluminum
d) Glass and stoneware
Answer: b
Clarification: Materials such as high-silicon cast iron, aluminum, glass, and stoneware are generally resistant to many acids but these are readily attacked in hydrofluoric acid. Whereas magnesium is practically immune to corrosion because of the formation of a surface fluoride film.

2. Hydrofluoric acid and fluorine are toxic and extreme care are mandatory in the handling of these materials.
a) True
b) False
Answer: a
Clarification: Hydrofluoric acid and fluorine are toxic and extreme care are mandatory in the handling of these materials. Metals such as magnesium are used for some shipping containers because of the formation of a surface fluoride film.

3. Which of the following metal is suitable in handling high concentrations (>60%) of hydrofluoric acid?
a) Stainless steel
b) Brass
c) Steel
d) Titanium
Answer: c
Clarification: Steel is suitable for handling hydrofluoric acid ranging concentration from 60% to 100%. The corrosion of steel rapidly increases as the concentration decreases below 60%.

4. Which of the following is/are the impurities in commercial hydrofluoric acid that decreases corrosion of steel?
a) Sulfuric acid
b) Fluosilicic acid
c) Neither sulfuric acid nor fluosilicic acid
d) Both sulfuric acid and fluosilicic acid
Answer: d
Clarification: Usually impurities such as sulphuric acid and fluosilicic acid are present in commercial hydrofluoric acid that reduces the corrosion of steel.

5. Which of the following metal is an outstanding material that resists all concentrations of hydrofluoric acid at all temperatures?
a) Wrought Monel
b) Steel
c) Stainless steel
d) Hastelloy C
Answer: a
Clarification: Wrought Monel is an outstanding metal in that it resists all concentrations of hydrofluoric acid at all temperatures including boiling. Monel is the best selection of material for hydrofluoric environment applications.

6. Why straight chromium stainless steels and austenitic stainless steels are not recommended for hydrofluoric acid applications?
a) Due to susceptibility to crevice
b) Due to limited corrosion resistance
c) Due to susceptibility to pitting
d) Sensitive to erosion-corrosion
Answer: c
Clarification: Straight chromium stainless steels and austenitic stainless steels are not recommended for hydrofluoric acid applications due to their susceptibility to pitting corrosion. Whereas (410) of stainless steel is sensitive to erosion-corrosion.

7. Which of the following materials is/are widely used for phosphoric acid?
a) 316 stainless steel
b) Durimet 20
c) Neither 316 stainless steel nor Durimet 20
d) Both 316 stainless steel and Durimet 20
Answer: d
Clarification: 316 stainless steel and Durimet 20 are the two alloys that are widely used for phosphoric acid. These alloys show the very little attack in concentrations up to 85% and temperatures including boiling.

8. Which of the following materials exhibits poor corrosion resistance to phosphoric acid?
a) Aluminum and brass
b) Aluminum, brass, steel, cast iron, ferritic and martensitic stainless steels
c) Steel and cast iron
d) Ferritic and martensitic stainless steels
Answer: b
Clarification: Metals such as aluminum, brass, steel, cast iron, ferritic, and martensitic stainless steels exhibit poor corrosion resistance to phosphoric acid. Whereas 316 stainless steel and Durimet 20 are widely used for phosphoric acid.

9. Which of the following is/are the impurities that cause corrosion in commercial phosphoric acid?
a) Fluorides
b) Chlorides
c) Sulfuric acids
d) Fluorides, chlorides and sulfuric acid
Answer: d
Clarification: Corrosion by phosphoric acid depends on the methods of manufacturing and impurities present in that commercial acid. Impurities that are usually present in commercial phosphoric acid are fluorides, chlorides, and sulfuric acid.

10. Which of the following non-metallic has the highest corrosive resistance to the phosphoric acid with temperature?
a) Teflon
b) Graphite
c) Polyvinyl chloride
d) Polyethylene
Answer: a
Clarification: Teflon is a non-metallic material that has the highest corrosion resistance to phosphoric acid up to 550°C. Teflon is also known as polytetrafluoroethylene (CF₂=CF₂).