Corrosion effect on a metal per unit of time. The type of corrosion rate used depends on the technical system and on the type of corrosion effect. Thus, corrosion rate may be expressed as an increase in corrosion depth per unit of time (penetration rate, for example, mils/yr. ) or the mass of metal turned into corrosion products per unit area of surface per unit of time (weight loss, for example, g/m-/yr. ). The corrosion effect may vary with time and may not be the same at all points of the corroding surface. Therefore. reports of corrosion rates should be accompanied by information on the type, time dependency, and location of the corrosion effect.