Inhibition is one of the most efficient methods of corrosion protection in many environments. However, most inhibitors provide synergitic properties when used in combination with other methods of corrosion protection. This report presents results of research on soluble inhibitors in combination with cathodic protection systems. Quoted parameters and critical concentration make corrosion prevention possible in different environments. It is established that inhibitors increase efficiency and longevity of protection. We have found that the consumption of inhibitors and the current of cathodic protection decrease 3-20 times. Soluble inhibitors together with cathodic protection systems should be used when corrosion protection is required in aqueous media, such as seawater, oil, gas, etc.
The corrosion rate of most structures produced from mild steel (e.g., ships, tankers, reservoirs, storage tanks, offshore platforms) used in aqueous media (e.g., seawater, oil, soil) depends on the aqueous solution compositions and different environmental conditions - concentration of CI, SO42, S 2, 02, pH, temperature, for example [1-3]. Over time, a corrosion rate could be changing from 0.05 to 0.30 mm/year, but in storage tanks and tankers for raw oil, for example, the aqueous solution on the bottom contains sulfide ions and the pitting corrosion rate can increase to 1-3 mm/year [1,2]. Obviously, corrosion protection systems must be reliable and efficient. These structures are using cathodic protection systems, coatings, and their combination [2-7]. Several compounds of inhibitors are known for these purposes [1,7], but the typical concentration needed for corrosion protection is very high or dangerous for the environment [1].
It is very difficult to choose a compound and a concentration of an inhibitor for such structures, because the composition and concentration of the media is not constant during the exposure time. Inhibitors should be used only in combination with additional corrosion protection methods. The best of them is a cathodic protection, whose efficiency can be adjusted to changing medium properties [2]. This paper shows that combining cathodic protection systems and inhibition provides very important synergistic effects.
EXPERIMENTAL
The specimens (50x20xl mm) were produced from strips of mild steel containing, in mass percent: 0.11 C, 0.94 Si, and 0.71 Mn. They were sand blasted, degreased in alcohol, cleaned by water flushing and dried in a stream of air or at room temperature in a glass cell with desiccant.
The corrosive medium had the following typical composition in mg/l: 27.7 NaC1, 0.2 NaHCO3, 3.4 Na2504, 1.1 CaCI2 and 2.4 MgC12. This electrolyte is some of the most corrosion active environment in sea water, soil, oil, and gas [1,2]. During the time of experiments, the pH was 7 to 8. The results seen in this medium are applicable for choosing corrosion protection methods for most structures in solution of aqueous nature [2,3].
The tested inhibitors were several different mixture with a concentration ratio of 1:1 between inorganic compounds (Na2H2PO4, ZnMoO4, ZnSO4, ZnC12) and organic acids and compounds with nitrite, phosphate, and chromate groups [CxHY NO2 - C13H26 NO2, CxHY N2CrO4 - C12H26 N2CrO4, CxHYN3(PO3)3 - CIIH28N3(PO3)3]. The concentrations were investigated in the range from 0 to 300 mg/L. This report shows only one example of the synergistic effect achieved with the following composition: 1:1 between inorganic (ZnSO4 + ZnMoO4) and organic [C12H26 N2CrO4]+ CllH28N3(PO3)3] inhibitors [ 1 ].
The electrochemical properties were investigated by potentiostatic cathodic polarization in the range from 50 to 300 mV
