Atmospheric corrosion and impact toughness of steels: Case study in steels with and without galvanizing, exposed for 3 years in Rapa Nui Island

Abstract

We studied atmospheric corrosion on Rapa Nui Island, using galvanized and non-galvanized SAE 1020 steel samples exposed on racks. We also added Charpy samples of both materials to directly determine the effect of corrosion rate on these materials' impact toughness. The results indicated a correlation between corrosion rate and toughness loss in the studied materials. In the corrosion study, we could also demonstrate the effect from increased insular population growth on contaminants which aid atmospheric corrosivity. Results showed that atmospheric SO₂ has tripled compared with similar corrosion studies done 20 years ago (Mapa Iberoamericano de Corrosión, MICAT), increasing corrosion rates. Our results show how human factors can influence changes in environmental variables that strengthen corrosion.

Keywords: Galvanized steel; Impact toughness; Marine environment; Mild steel; atmospheric corrosion.

Fig. 1

(a) Charpy (Simple-Beam) impact test specimen, Type A [46], (b) Results from Charpy impact testing of different maraging steels, tested at 80 °C and – 30 °C [48].

Fig. 2

(a) Atmospheric testing station in Rapa Nui, (b) Devices installed at the station to detect Sulfur dioxide, and (c) chloride deposition rates.

Fig. 3

Photographs of the racks used in the studies, showing the upper section of the rack, with the samples for corrosion rate (a and c), and the lower section, with hung samples for impact toughness tests (b and d).

Fig. 4

Monthly maximum and minimum temperatures measured for three years in Rapa Nui.

Fig. 5

Maximum and minimum monthly relative humidity measured for three years in Rapa Nui.

Fig. 6

Pollutants measured in Rapa Nui during different study months, (a) chloride content and (b) SO₂ content.

Fig. 7

Corrosion rate (a) and thickness loss (b) of carbon steel, as a function of exposure time.

Fig. 8

Corrosion rate (a) and thickness loss (b) of galvanized steel, as a function of exposure time.

Fig. 9

SEM micrographs of two carbon steels exposed for 1 and 3 years in Rapa Nui (100×). (a–b) surface image and cross-section of the sample after 1 year of exposure, and (c–d) surface image and cross-section of the sample after 3 years of exposure.

Fig. 10

X-ray diffraction analysis (XRD) of samples after 3 years of exposure time: (a) steel, where the main components were Lepidocrocite and Goethite, (b) galvanized steel, where the main component was simonkolleite.

Fig. 11

SEM micrographs of two galvanized steels exposed to the environment on Rapa Nui (100×). (a–b) surface image and cross-section of the sample after 1 year of exposure, and (c–d) surface image and cross-section of the sample after 3 years of exposure.

Fig. 12

Anodic curves for unexposed carbon steel, with 1 and 3 years of exposure.

Fig. 13

Anodic polarization curve of unexposed galvanized steel exposed to 1 and 3 years.

Fig. 14

Photographs of the racks used in the study stations, showing (a–b) the upper section of the rack and the samples exposed to determine corrosion rate, and the hanging samples for impact toughness tests (c–f) after 2 years' exposure in a C3 environment.

Fig. 15

Steel toughness obtained at different exposure periods.

Fig. 16

Graphic comparison between corrosion rate and fracture toughness loss in steels.

Fig. 17

(a) Microstructure of galvanized steel used in the study, showing the phases composing the galvanized layer, magnified 50×, (b) Zone rich in Zn, of the phase diagram in equilibrium Zn-Fe [73].

Fig. 18

Toughness of galvanized steels obtained at different exposure periods.

Fig. 19

Graphic comparison between corrosion rate and fracture toughness loss in galvanized steels.