Experimental Investigation on the Corrosivity of Atmosphere through the Atmospheric Corrosion Monitoring (ACM) Sensors
Authors: Shinohara Tadashi , Dara To , Umezawa Osamu 
Published: February 2020

Atmospheric corrosion is an essential problem to industrial and infrastructure engineering. The abundant concerned of the problem is the lifetime service of metal structure and equipment which result in the safety operation and economic loss. Atmospheric corrosion is a very complicated process that causes deterioration of metallic materials by chemical or electrochemical reaction between the metal and its environment. The corrosion parameters like relative humidity, time of wetness, and numerous pollutant substances which involve species deposited from atmosphere and species from the metal resulting from corrosion itself, affect to the atmospheric corrosion.

A common method for atmospheric corrosion investigation is the long-term exposure test under natural environment, but this method requires time-consuming and high-cost expanse. To investigate the corrosivity of atmosphere, in this research, several chemical species such as NaCl, KCl, Na2SO4, NaNO3, KNO3, MgCl2.6H2O, and Mg(NO3)2.6H2O were used as artificial rainfall solution with the variation of concentration. Four types of ACM sensors which consist of Fe-Ag, Zn-Ag, Al-Ag, and Cu-Ag galvanic couple were used to define the correlation between sensor output and Corrosion Rate (CR), which affected by the chemical concentration of individual and mixed solutions. The 1 mm thick of low-carbon steel sheet specimens, with the size of 70 × 150 mm, were exposed to the artificial rainfall test. The CRs were calculated by measuring weight loss after removing corrosion products, or rust layer, by immersing the samples into Hydrochloric Acid (HCl) solution (
ISO/DIS 8403.3).

In water, the molecules of the species are separated into cation and anion. The results reveal that the corrosivity of the cation is negligible compared to the anion. In term of the concentration, the SO4
2- anion is more corrosive than Cl- and NO3
- anion respectively. For a low chemical concentration, the CRs affected by each ion are small and similar to the CR of Deionize-water. This is the result of the spontaneously formed of the oxide film which provides some degree of protection, and the breakdown of the film is usually required higher aggressivity of the electrolyte. For further increasing the molar concentration, the CR raises up to reach a steady state.

The signal output of Fe-Ag and Zn-Ag couple give a good relation for determining the corrosion rate in both individual and mixed solutions. Al-Ag couple, however, is unable to forecast the CR in the unknown pollution species, especially the mixed solution, because it is more sensitive with Cl- and NO3
- ions and less so with SO4
2- ions. Nonetheless, Al-Ag galvanic sensor offers a great advantage to predict the chemical species in the environment. For instance, if the CR is high, and the Al-Ag sensor output is low, the SO4
2- ions is assumed as the major contain in the solution or the environment. The lifetime service of Fe-Ag galvanic sensor is much lower than those of Zn-Ag, Al-Ag, and Cu-Ag, respectively.

The results of the research indicate that the ACM sensors are capable of estimating corrosivity of the atmosphere; the research methods discussed in this paper proves that the CRs are dependent on the atmospheric composition and can be forecasted through ACM sensors.