Asset

Depreciation is reduction in value of an asset due to use , effluxation (passage of time) , obsolesence.(Normal reasons)

Impairment of asset is generally due to damage caused by fire, flood, short circuit, etc (Abnormal reason generally)

Regards

When an assets' Carrying Amount (net after dep.) is less than its Recoverable Amount, then there exists an Impairment and accordingly Impairment loss is to be recognised.

What is Recoverable Amount ?

Higher of

Net Selling Price - Fair value less disposal cost

Value in use - Present value of estimated future cash flows that would flow from continuing use of the asset.

Depreciation is the normal amortisation of asset value reduced by salvage value. This amortisation is due to normal usage of the asset to match its charge against the matching revenue. While impairment is reduction in the revenue / cashflow generating capacity due to some internal or external factors.