Passivating layer on alloy is dissolved completely by:

 # Passivating layer on alloy is dissolved completely by:
A. Basic acid
B. Borax
C. Potassium fluoride
D. Graphite

The correct answer is C. Potassium fluoride. 

# Aluminum, chromium, and titanium have high affinity for oxygen and they form protective oxide film, which prevents further oxidation and corrosion. This is called passivation. For soldering these alloys, the passivating film should be dissolved. Potassium fluoride used as a flux dissolves passivating film so that the solder can wet the parts. 

# Flux is a compound applied to metal surfaces that dissolves or prevents the formation of oxides and other undesirable substances that may reduce the quality or strength of a soldered or brazed area. 

# There are three types of fluxes according to their primary purpose: - 
Type I: Surface protection- covers the metal surface and prevents access to oxygen so that no oxides can form. 
Type II: Reducing agent- reduces any oxides present and exposes clean metal. 
Type III: Solvent- dissolves any oxides present and carries them away. 

Fluxes for use with noble metal alloys are generally based on boric acid, boric anhydride, and borax. Chromium oxides that form on base metal alloys are more stable and fluoride containing fluxes are used as solvents to dissolve chromium, nickel, and cobalt oxides. 

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