Zinc against corrosion

Protecting iron from corrosion

Safe­ty pre­cau­tions

Wear pro­tec­tive gloves, eye­wear, and a mask. Per­form this ex­per­i­ment in a well-ven­ti­lat­ed area.

Reagents and equip­ment

  • 10 g sodi­um chlo­ride;
  • 80 mL dis­tilled wa­ter;
  • 2 zinc wash­ers;
  • 2 pieces of iron;
  • 2 plas­tic cups;
  • 5 mL 5% am­mo­ni­um thio­cyanate so­lu­tion;
  • 10 mL 10% hy­drochlo­ric acid.

Step-by-step in­struc­tions

Dis­solve 5 g sodi­um chlo­ride in 40 g of dis­tilled wa­ter in 2 plas­tic cups. Add 2 zinc wash­ers to the first cup. Im­merse one piece of iron in each cup. Make sure the iron is touch­ing the zinc in the first cup! Wait 40 min­utes. No­tice that the so­lu­tion in the sec­ond cup has turned or­ange. Con­duct a qual­i­ta­tive re­ac­tion for iron ions: add 10 mL of 10% hy­drochlo­ric acid so­lu­tion to the sec­ond cup, then add a few drops of 5% am­mo­ni­um thio­cyanate so­lu­tion to both cups. No­tice that the so­lu­tion in the sec­ond cup turns red.

Process de­scrip­tion

Cor­ro­sion is a process by which ma­te­ri­als (met­als, plas­tics, wood, con­crete, and so on) are grad­u­al­ly chem­i­cal­ly de­stroyed by en­vi­ron­men­tal fac­tors. Cor­ro­sion car­ries a dev­as­tat­ing eco­nom­ic loss, speeds ag­ing, and brings de­struc­tion. Ca­thod­ic pro­tec­tion is one method used to fight cor­ro­sion. The met­al be­ing pro­tect­ed is at­tached to a more ac­tive met­al. This ex­per­i­ment demon­strates the de­fense of iron by zinc in a sodi­um chlo­ride so­lu­tion im­i­tat­ing sea­wa­ter. The zinc dis­solves, and the iron is left un­touched.

Zn - 2e = Zn²⁺

Iron cor­rodes in the so­lu­tion with­out zinc. It dis­solves, form­ing iron(III) oxy­hy­drox­ide:

Fe + 2H₂O - 3e = FeO(OH)↓ + 3H⁺

A qual­i­ta­tive re­ac­tion with thio­cyanate ions can be used to prove the pres­ence of iron(III) ions – a red com­plex will form. Di­lut­ed hy­drochlo­ric acid is added to dis­solve the or­ange pre­cip­i­tate of iron(III) oxy­hy­drox­ide, then a few drops of am­mo­ni­um thio­cyanate are added.

FeO(OH) + 3HCl = Fe­Cl₃ + 2H₂O

Fe³⁺ + nSCN⁻ = [Fe(SCN)ₙ]³⁻ⁿ

Such a “zinc de­fense” can serve to save ships from cor­ro­sion in such en­vi­ron­ments as sea­wa­ter.