WHY DOES DI HAVE A LONGER LIFE CYCLE THAN STEEL?
- Corrosion Mechanisms: Corrosion occurs in both DI and steel, and the corrosion process in both systems requires oxygen. But beyond that point, the corrosion mechanisms for each material are very different.
- For DI the dominant corrosion mechanism is surface oxidation. A uniform oxide layer forms and permanently bonds to the DI surface; and, over time this oxide layer reduces the amount of oxygen that can reach the metal surface, reducing the rate of corrosion. Corrosion of DI is a gradually diminishing process over time.
- For steel there are two corrosion mechanisms: localized “pitting”; and, the formation of a loose scale of iron oxide, sulfates and nitrates on the steel surface. Over time layers of this loose scale drop off; and, are replaced by new layers of loose scale, formed in an on-going process. Corrosion of steel is a continuous process.
- Corrosion Rates: Corrosion Rates for all materials are site specific, defined by the corrosivity of the soil in which the material is imbedded. At one corrosivity level, rates are: 0.8 mil/year for DI, and 3.0 mils/yr for steel. Absolute rates often vary from site to site, caused by changing corrosivity levels. But, many field tests have shown that the differential between these corrosion rates remains relatively constant at all corrosivity levels: Steel corrodes 3-4 times faster than DI.