Improvement of surface properties of hot-dip aluminized 316Ti grade stainless steel by using micro arc oxidation process

Micro arc oxidation (MAO) is a surface modification technique that transforms surfaces of valve metals such as Al, Mg, Ti, etc. into ceramic coatings. It is also known as plasma electrolytic oxidation (PEO), micro plasma oxidation and spark anodizing. In general, it is considered to be inexpensive and simple processing technique. However, steel substrate materials are not favorable to use directly for surface modifications. In order to use the advanced surface modification technology for steel surface, researchers have tried to use indirect ways to get ceramic coatings on steels. For instance, MAO process was performed after hot-dipping or arc spraying aluminum layer on steel surfaces was produced. Compared to the arc-sprayed aluminum layer, the hot-dip aluminum layer has higher bond strength to steel surfaces because of the metallurgical bonding between the aluminum layers and the steel substrate. Therefore, the hybrid method of micro-arc oxidation after hot-dipping aluminizing (SDA) is very promising to prepare protective composite ceramic coatings on the steel surface. A lot of conventional surface techniques such as arc spraying, hot-dip aluminum, and plasma sputtering aluminum are applied to employ aluminum layer on steel to act as anodic barrier layers. When hot-dipped aluminum layer is compared to arc sprayed aluminum layer, it has higher bonding strength to steel substrate thanks to metallurgical bonding. Therefore, the hybrid method of SDA before MAO has a promising future surface modification technology on steel substrates. Firstly, MAO method is described in details. The effective factors of MAO process are voltage, current density,electrolyte, duty cycle and processing time in order to fabricate ceramic coating. These factors have a lot of advantages to change surface properties easily and produced as required. It is an oxidation process which contains basically anodic oxidation. The anodic oxidation cells are composed of anode, cathode, electrolite and power supply. While the cathode material is stuck to the negative electrode, the anode material insert positive electrode in oxidation. In MAO process, sparks are seen obviously on the substrate material of anode when the breakdown voltage is overpassed. At the beginning of MAO process, the temperature of sparks increases high temperature such 104 K. After then, due to electrochemical reactions between cathode and anode material, oxidation reactions occur on the surface of the anode. In the course of growing oxide layer, molten material coming from discharge channels freezes on oxide layer. Therefore, it gets thicker with increasing oxidation duration. Hot-dip aluminizing process is widely utilized for coating aluminum and its alloys on steels. The method involves dipping of steel samples into molten aluminum or its alloys, at a fixed temperature, for a specific period of time. In hot-dip aluminizing, the Al/steel interface consists of Al-rich intermetallics compouns of Fe2Al5 and FeAl3 in general. These intermetallics compouns are brittle and brought about cracking and peeling while bending and machining. It is always desirable to keep the minimum intermediate compound layer as the intermetallic alloys in this layer are very brittle in nature. Hot-dip aluminizing is also the popular surface modification technology for protection of iron-base materials. In the literature review, many studies on aluminum coatings by hot-dip process are published. The process of hot-dip coating formation should rather be regarded as a type of controlled corrosion process. This idea as a starting point for further research, it focuses on phenomena and processes occurring in the Fe-Al interface. In hot-dip aluminizing process, commercially pure aluminum is poured into stainless steel crucible. Hot dip furnace is heated up to 700 °C to melt aluminizing bath. Aluminizing was carried out at 700 and 800 °C for 5, 10 and 20 minutes. After aluminizing process, MAO process was applied to the aluminized steel in an alkaline electrolyte to generate a hard ceramic coating at the surface. The electrolytic solution is mainly composed of NaAlO2 (15 g/L) prepared with deionized water and KOH (2 g/L). The micro arc oxidation parameters are positive and negatvie voltages of 450 V+/ 80 V- and 500 V+/ 90 V- and oxidation time of 5 minutes. Microstructural and morphological characterizations of the coating were conducted by cross-sectional optical microscopy and scanning electron microscopy (SEM) examinations, respectively. Successive application of these two techniques results in the formation of three-layer coating at the surface, namely, a Fe-Al intermetallic rich inner layer close to the substrate, an aluminium rich intermediate layer and an alumina top layer. The ceramic coatings exhibits a typical prous morphology of the MAO process. The surface has a lot of micro-cracks. It is attributed that the development of such defects in the MAO coatings is due to to the thermal stresses during the coatings evolution, which are resulted from melting–solidifying and discharging, and some other fast heating–cooling processes all over the growth process of the ceramic coatings. Acquired phases at the surface were qualitatively identified by X-ray diffraction analyses. The constituent phases of intermetalic layer in the dipping state consisted of Al, FeAl3 and Fe2Al5 compounds. At relatively low dipping temperatures, the proportion of FeAl3 is higher than Fe2Al5, but with increasing dipping temperature, the amount of Fe2Al5 increased. The micro arc oxidation ceramic coating of aluminized 316Ti grade stainless steel in NaAlO2 electrolytic solution is mainly composed of α-Al2O3, β-Al2O3 and Al. Micro hardness measurements were performed on the cross-sectional of MAO coating sample. Hardness tests reveal that the microhardness of substrate material is 214 HV0.025, aluminized coating is 51 HV0.025 and MAO coating 1043 HV0.025, respectively. The microhardness of the ceramic coating is almost five times as that of the uncoated substrate material. The corrosion resistances of 316Ti grade stainless steel before and after micro arc oxidation and hot-dip aluminizing were tested through Gamry Reference 600 electrochemical corrosion testing system. At the room temperature of 24ºC, the corrosive medium is 3.5 wt. % NaCl solutions, reference electrode is saturated calomel electrode, auxiliary electrode is Pt electrode, and corrosion time is 1.5 h. The corrosion resistance of aluminized 316Ti grade stainless steel surface is obviously improved in comparison with the substrate material and MAO samples. The electrochemical corrosion experiment shows that the corrosion resistance of aluminized material is improved by two times comparing with the substrate material. Moreover, the friction coefficient of original and coated substrate on a reciprocating sliding tribometer. The friction coefficients versus sliding time for the coated and uncoated samples were plotted. The curves show that the friction coefficient for MAO sample was in the range of 0.25 to 0.35, whereas this value was approximately 0.45 for the aluminized sample, and the latter was more stable. Lower friction of the coated sample may be attributed to the worm structure of the MAO coating. The high hardness of the coatings was favorable to the better wear resistance. Moreover, the dense structure of the layer might considerably improve the load-bearing capacity of the coatings. The wear tests at dry sliding conditions showed that MAO'ed surface on aluminized layer exhibited superior wear resistance while sliding against alumina counter-material.