Effects of naturally-progressed corrosion on the chemical and mechanical properties of structural steels
Introduction
Steel is a common material used for the construction of naval, offshore, mechanical, and civil engineering structures. As steel structures get older, their safety and integrity can suffer from corrosion wear which is affected by various parameters of influence in the corrosive environment, including oxygen content, salinity, pH value of water, temperature, atmospheric pressure, suspended solids, velocity of water waves, together with various physical and chemical factors of material [1], [2], [3], [4], [5]. Land-based steel structures may be exposed to the immersion of freshwaters or related humidity while the surfaces of steel ship and offshore structures at sea are usually touched on seawaters [6]. In winter season or Arctic area, the operational temperature of such structures is in sub-zero temperatures (or lower than the room temperature).
To evaluate the structural integrity with corrosion damage at the level of steel structural members or entire structures, it is essential to identify the chemical and mechanical properties of corroded structural steels, i.e., at the level of materials. It is generally considered that corrosion does not affect the chemical and mechanical properties of structural steels [3], but obvious evidences or test database are lacking in the literature and thus some studies attempted to derive computational models that the corrosion wear was dealt with as a parameter of influence on the mechanical properties of structural steels [7], [8], [9], [10], [11]. The objective of this paper was to obtain the physical test database to investigate the effects of corrosion wear on the chemical and mechanical properties of structural steels, and ultimately to contribute to the prevention of such an unnecessary confusion on the issue.
The progression of corrosion with time depends on the corrosive environment and it is usually not very fast by nature taking several months or years. Structural steels with coating on surface do not commence corrosion until coating fails [12], [13], [14]. The corrosion progression characteristics are obviously probabilistic and random with not only time but also various sources of uncertainties, as found from pioneering works of the Paik’s group with probabilistic models to predict corrosion wastage in terms of both time and probability density distributions [15], [16], [17].
Physical tests on corrosion of structural steels have been conducted at an artificial condition of the corrosive environment that could accelerate the corrosion progress and produced a desired quantity of corrosion in the laboratory [18], [19], [20], [21]. These approaches were able to save testing time and enhanced the work efficiency during the corrosion progress. Other corrosion test studies are available with the corrosion conditions which are similar to actual fields of operation [22], [23]. Most corrosion test studies in the literature have been focused on how the corrosion progresses and what the corrosion rate is [1], but the related studies on both the chemical and mechanical properties of structural steels are not found in the literature.
In this paper, corrosion tests on the specimens of mild steel (grade A), AH32 steel, and DH32 steel were conducted for a period of 12 months. The shape and geometry of test specimens were exactly the same as for the coupons which were used for tensile testing to examine the mechanical properties of steel. The specimens for the chemical property tests were also prepared and tested. The glow discharge spectrometer [24] was used for the chemical property tests, and the hydraulic universal testing machine was used for the mechanical property tests.
The present study was focused on the effects of corrosion on the chemical and mechanical properties for three different grades of structural steels. Three dry or water-immersed conditions were considered: air, freshwater immersion, and seawater immersion. The mixture of dry and water-immersed conditions as in case of water ballast tanks of ships was not considered. Three temperature conditions were considered: room temperature (18 °C) and two cold conditions with a temperature of 0 °C and −10 °C. Any artificial acceleration of the corrosion was not attempted, but rather the corrosion was naturally progressed during a period of 12 months. The chemical and mechanical properties of test specimens were measured before and after the corrosion testing. Based on the test results, the characteristics of the corrosion progression rate were studied and reported in a separate paper [25].
Section snippets
Types of specimens
Two groups of test specimens were prepared, i.e., one for mechanical properties and the other for chemical properties.
For measuring the mechanical properties, the corrosion test specimens were fabricated in exactly the same shape as for tensile coupon test specimens as shown in Fig. 1, where the geometric specifications of the American Society for Testing and Materials (ASTM) E8 was applied [26], with a gauge length of 60 mm, and thickness 6 mm. The mechanical properties of the test specimens
Methods for corrosion tests
Naturally-progressed corrosion was allowed to develop in different dry or water-immersed conditions by keeping the tensile specimens in fully submerged or in open air at different temperatures. Two specimens from each steel grade were immersed in three dry or water-immersed conditions, namely air (dry), freshwater immersion and seawater immersion.
Test results on the chemical properties of corroded specimens
The chemical composition of mild steel (with grade A), AH32 steel, and DH32 steel were measured after the progression of corrosion in seawater submerged condition at a temperature of 18 °C, 0 °C and −10 °C. The motivation of the present paper was initiated to acquire the test database on the corrosion of ship structural steel exposed to the corrosive marine environment which may affect the chemical composition of steel [28]. Maximum corrosion was expected to be observed in seawater submerged
Test results on the mechanical properties of corroded specimens
As reported in [25], maximum corrosion wastage was observed in all the three types of specimens at 18 °C (warmest) under seawater submerged condition. AH32 and DH32 steel experienced faster corrosion progress rate than mild steel.
The mechanical properties of corroded steels were identified through quasi-static tensile testing. It should be noted that due to corrosion the surface of specimens become uneven because of the formation of micro-pits on the surface, as illustrated in Fig. 4.
Concluding remarks
The aim of this paper was to experimentally examine the effects of corrosion wear on the chemical and mechanical properties of structural steels with the varying corrosive environments such as dry or water-immersed condition and cold temperature. Three kinds of structural steels with different grades were tested. In addition to dry (air) condition, two kinds of water-immersed conditions consisting of freshwater and seawater immersion were considered. The corrosion tests were continued for a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was conducted at the International Centre for Advanced Safety Studies (The Korea Ship and Offshore Research Institute, www.icass.center) which has been a Lloyd’s Register Foundation Research Centre of Excellence since 2008.
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