Define corrosion, how you will join the metals to prevent corrosion, justify with diagram.

Corrosion is the gradual destruction or deterioration of a metal by chemical or electrochemical attack from its environment, starting at the surface. Example: rusting of iron, Fe → Fe₂O₃·xH₂O.

Joining metals so that corrosion is minimised (proper design):

• Avoid contact between two dissimilar metals. In the presence of an electrolyte they form a galvanic cell, and the more active metal (the anode) corrodes.
• If dissimilar metals must be joined, choose metals that are close together in the galvanic series, and place an insulating washer or gasket (rubber, plastic) between them so no galvanic cell forms.
• Keep the anodic area large and the cathodic area small. For example, use copper rivets on a steel plate, not steel rivets on a copper plate.
• Prefer welding to riveting or bolting, because welding leaves no crevices where moisture can collect (crevice or differential-aeration corrosion).

  Wrong: direct contact               Right: insulated joint
      bolt (Fe)                          bolt (Fe)
   ====|====  Cu plate                ====|====  Cu plate
   ====|====  Fe plate                ----|----  insulating washer
       nut                            ====|====  Fe plate
  -> galvanic cell, Fe corrodes           nut  -> no electrical contact

Justification: corrosion needs an anode, a cathode, an electrolyte and an electrical path. An insulator breaks the electrical path. A large anode area keeps the current density at the anode low, so corrosion is slow.