1. Introduction
Magnetorheological elastomer (MRE) possesses excellent rheological properties in which their stiffness can be controlled continuously and reversibly in the presence of an external magnetic field. MREs consist of micron size of magnetic particles, particularly Fe or carbonyl iron particles (CIPs) embedded in a non-magnetic matrix. Generally, natural rubber (NR) [
1,
2], silicon rubber (SR) [
3,
4], epoxidized natural rubber (ENR) [
5] and thermoplastic [
6] are utilized as matrix materials in the fabrication of an MRE. During the last two decades, MREs had been receiving considerable attention from both the academy and various fields industry owing to their enormous potential in engineering applications, particularly in the research and development of MRE devices such as bushing [
7], vibration isolators [
8], base isolators [
9] and sensing devices [
10,
11,
12].
Conventional rubber components such as vibration isolator and damper are frequently used for a long period of time and are subjected to various temperatures or/and under severe environmental conditions such as high oxygen, water, and oil contamination exposure. As a result of being exposed to these conditions, the rubber in engineering components may suffer from environmental degradation. The exposure can profoundly affect its level of performance [
13] by changing the quality and characteristics of the material components, hence shortening its useful operation time [
14,
15,
16,
17,
18]. Zhu et al. [
16] fabricated two types of rubber, namely hydrogenated nitrile-butadiene rubber (HNBR) and nitrile rubber (NB), in order to investigate the effects of CO
2 to the corrosion phenomenon under actual downhole condition. They found that the hardness of the rubbers decreased by up to 15% under a compression state for 168 h. Morphological changes were also observed with the appearance of shallow and small-sized holes on the surface of the HBNR. Sarlin et al. [
17] had investigated the environmental resistance of stainless steel/rubber/glass fiber reinforced epoxy (GFRP) hybrid structures under the exposure of hot, moist and hot/moist environments. In this particular test, the researchers used different grades of EPDM-based rubber with the following components; polyethylene wax (rubber A), silica (rubber B) and carbon black (rubber C). After having undergone the process of hygrothermal exposure (85 °C, 85%RH), some minor degradations were observed in rubbers B and C at 370 °C, while rubber A showed symptoms of degradation at 500 °C. At the same time, the hardness level of rubbers B and C was increased by 5%–7% as compared to rubber A. In another study, Ozawa et al. [
18] discovered a weakened interfacial adhesion that was caused by the increased of oxygen composition in the rubber-brass interface. Thus, for this reason, study on the degradation of material properties, as a result of hygrothermal aging or under practical conditions, is essential in the designing stage of material structures.
Nonetheless, magnetic particles such as Fe, Ni, Co and NdFeB are widely used in various fields such as electronics, acoustics, automation and biomedical applications. Song et al. [
19] found HNO
3 to be the strongest corrosion electrolyte that obviously accelerated the corrosion of NdFeB in their study about corrosion behavior in various electrolyte solutions such as NaOH, NaCl, HNO
3, and H
2C
2O
4. Pores and crevices were observed in the morphology of NdFeB after being immersed for 0.5 h in HNO
3, hence implying a degradation of magnetic properties from the hydrogen released in the magnets’ surface. On the other hand, Dunn et al. [
20] proposed in-situ alternating current (AC) and direct current (DC) electrochemical techniques in determining the corrosion rate and corrosion potential of high-purity iron under alternative wet and dry states.
It is a well-known that magnetic particles are the main component in the MR materials such as MR fluid, MR grease, MR foam and MRE. The study on the effect of corroded carbonyl iron particles (CIPs) in MR fluid has been reported by a few researchers. Han et al. [
21] studied the effect of corroded CIPs on the performance of MR fluids, particularly in the field-dependent properties such as the yield stress. They found that the shear stress of MR fluid with the corroded CIPs decreased up to 11% and with a slower response time than those of the non-corroded MR fluid. In studying the influence of 0.05 HCl on the corrosion of CIPs in MR fluid, Plachy et al. [
22] discovered the formation of the Fe
2O
3 oxide layers on the surfaces of the particles, while the magnetic saturation of the oxidized CIPs decreased up to 35% more than non-oxidized CIPs. Recently, Cvek at al. [
23] fabricated two types of CI particles grafted with poly(trimethylsilyloxyethyl methacrylate) (PHEMATMS) of two different molecular weights. The results revealed that the PHEMATMS did enhanced the stability of CI particles and prevented the degradation in an acidic condition.
As evident from the abovementioned literature, attention and majority of the published works was mainly focused to the influence of corroded CIPs solely in rheological properties of MR fluid. However, the effect and the influence of the corroded iron particles on the physicochemical and rheological, properties particularly the storage modulus and damping of MRE has not been reported so far. Thus, further investigation and the mechanism of the corroded CIPs effect toward the properties of MRE needs to be undertaken through both the morphological and rheological approaches. For this purpose, in this work an attempt is made to characterize and emphasize comprehensive physicochemical and rheological properties of MRE samples with non-corroded and corroded CIPs through empirical methods. Therefore, it is asserted that the main technical contribution of this work is to experimentally investigate the principal characteristics of MREs such as the field-dependent storage modulus by fabricating MRE samples containing the corroded CIPs. This research idea comes from the possibility that new iron particles containing in MRE may be corroded after long-time use of MRE under practical conditions. This investigation is closely related to the behavior of MRE applied under various environmental conditions and hence is a very significant area to be explored. In this study, hydrochloric acid (HCl) is used to produce corroded CIPs for the fabrication of silicone rubber based MRE samples. The correlation between microstructure evolution, magnetic behavior and rheological properties such as storage modulus, damping properties and MR effect of the corroded CIPs in MRE is systematically investigated.