No. Scrap sales are not liable for NBT. If you go through the liable TO under the NBT act,Liable TO purely includes the sales from your busines. Hence Scrap sale is not your business, it is not chargeable under NBT act.
http://www.ird.gov.lk/en/Type%20of%20Taxes/SitePages/Nation%20Building%20Tax(NBT).aspx?menuid=1206