Conventional diffusional heat treatments are usually performed when an increase on the surface hardness of a certain component is desired. However, some alloys, such as duplex stainless steels and nickel alloys cannot be submitted to the temperature ranges that these treatments require due to the precipitation of deleterious phases. Besides that, since these alloys are used in highly corrosive environments, the effect of those treatments on corrosion resistance is a point that deserves special attention.
This paper presents a study about the effect of two different diffusional heat treatments on the corrosion behavior of two nickel alloys (UNS N07718 and UNS N09925). The first one (HT#1) can be classified as a carburizing process, and the second (HT#2) as a nitriding process. Treated and untreated samples were analyzed through a corrosion test called critical pitting temperature (CPT) that was determined through an electrochemical method, according to ASTM G150, and by a chemical one, per ASTM G48. The results showed that the heat treatments presented opposite effects on the substrates. HT#1 improved the corrosion resistance of both substrates, although 925 HT#1 presented a lower CPT value by ASTM G48, which can be explained due to the test method aggressiveness, and HT#2 reduced their CPT. In all cases where pits were observed, they were macroscopic.
Austenitic stainless steels have excellent corrosion resistance in various environments so they are used as essential material for several parts such as pipeline pumps and structural steel in many industries. Among them 316L stainless steel has good mechanical properties with high corrosion resistance; therefore it is regarded as the one of the most effective materials in various corrosive filed. However in marine atmospheric environments it could be suffered pitting-type corrosion due to seawater and deposition of airborne sea salts which has high chloride concentration. For this reason corrosion protection in high chloride environments is the major concern in many industries at the same time several corrosion mitigation methods are studied and developed in various fields.Vegetable oils are highlited as new effective organic coating which is a nontoxic nondepletable domestically abundant nonvolatile and biodegradable resource. Polar molecules present in the oils can be absorbed on metal surfaces and form the corresponding metal oxides which will enhance the stability of passivation and promote adhesion. Especially sunflower oil (SunFO) coating is reported as a effective corrosion inhibition organic film for carbon steel. However there is seldom studies for corrosion inhibition effect of the sunFO coating for stainless steel and reinforcement effects of the sunFO coating due to synthesis of other materials. Therefore in this study the sunflower oil is used as base coating and binding materials for 316L stainless steel.Recently the two-dimensional (2D) material is extensively researched for the modification of physical properties such as a mixure materials with polymeric organics because they affect corrosion resistance. Among the 2D meterials MoS2 is spotlighted becaues of semiconducting property between the metal and electrolytes. However many modifications such as CVD water transfer for surface coating with MoS2 are being phased out due to their high cost long time in process low throuput and harmful effects on environments. For many researches were conducted to improve stability and corrosion resistance with a mixture between the MoS2 particles and other organic coating material without the toxicant effects.In this study the reinforcement effect of MoS2 in organic coating on 316L stainless steel was investigated in 3.5 % NaCl solution. The electrochemical properties of organic coating according to MoS2 were evaluated by using electrochemical impedance spectroscopy (EIS) tests. Scanning electron microscopy (SEM) with Energy Dispersive Spectrometer (EDS) was used to observe the dispersion of MoS2 on the surface and cross section. In addition X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) methods were performed to evaluate corrosion resistance of coating materials.
Magnesium ammonium phosphate hexahydrate (MgNH4PO4·6H2O Struvite) scale deposits is often a serious operational problem in anaerobic treatment of municipal waste water and other industrial wastewaters. The kinetics of the nucleation and crystal growth at conditions simulating municipal wastewater was investigated in a 1L reactor at conditions of constant supersaturation at pΗ 9.50 25°C both in the absence and in the presence of As(V) and Hg(II) at concentration levels precluding the formation of insoluble salts of these impurities. Over the range of the relative supersaturation with respect to struvite investigated (σ=0.096-0.37) the presence of As(V) resulted in lower rates of crystal growth while the mechanism was the same in both cases i.e. surface diffusion controlled. The effect of As(V) concentration on the crystal growth rates of struvite was diminished as the supersaturation increased. The presence of As(V) at concentrations in the range 0.1-1 mM resulted in 90% reduction of the rates of crystal growth while induction times increased significantly. The presence of Hg(II) resulted in acceleration of the rates of crystal growth while no induction times were observed. In both cases equilibrium adsorption measurements showed that the effect of As(V) and Hg(II) is due to the interaction of these ions with the surface of the growing supercritical struvite nuclei.