ABSTRACT Some natural gas distributors for residential and commercial buildings in Brazil use galvanized steel tubes and fittings manufactured by the hot-dip zinc process that are supported by the specifications contained in the Brazilian Standards. Zinc coating is one of the most widely used methods to provide protection against corrosion. It is therefore essential that national and international tube manufacturers and installation and maintenance companies in civil construction also meet the defined national and international standards, especially when the tubes and fittings are used for natural gas in residential buildings. This two-way communication could eliminate most issues that delay or prevent superior galvanizing quality. Low standards lead to the deterioration of the zinc coating and the metal base of the pipes, which may lead to leaks, such as fluids, flammable gases, and other toxic products, and potentially jeopardise residential and commercial buildings. Here the importance of ensuring the quality of zinc-coated pipes is demonstrated through a series of analyses on natural gas pipe corrosion used in civil construction in Brazil. Based on Brazilian standards, the zinc mass determination per square meter, uniformity tests (Preece test) and metallographic evaluation of the deposited zinc layer were carried out on samples of galvanized tubes for natural gas. An experiment was also developed to reproduce the same conditions of subfloors (mortar beds) where there is penetration of water and humidity reaching the galvanized pipes. Metallographic analysis showed that samples of galvanized tubes by the hot-dip zinc galvanizing can have discontinuities and defects in zinc layer. Such defects associated with penetration and/or permeation of the water and humidity in the subfloor favor the corrosive process and consequently can cause gas leaks and probable accidents in residential buildings.

The AISI 317L stainless steel is an austenitic grade with at least 3%Mo. Recently, this steel has replaced AISI 316L in many applications in chemical and petrochemical industries, due to its higher pitting corrosion resistance. The microstructure of the hot rolled and annealed material studied in this work consists of austenitic grains and 4.0% of delta ferrite (δ) with elongated islands morphology. This microstructure was modified by three processes: cold rolling with 87% of reduction, aging at 450ºC for 400 h, and welding by gas tungsten arc welding (GTAW) process. The corrosion resistance was evaluated by anodic polarization tests (ASTM G-61) and critical pitting temperature tests (ASTM G-150). Cold rolling produced a microstructure consisting of elongated grains of austenite and martensite α', high dislocation density and the elongated islands of delta ferrite present in the annealed material. Welding produced a dendritic microstructure with 7.0% of delta ferrite and some σ precipitated in the ferrite. Finally, the aging at 450ºC for 400 h provoked the decomposition of ferrite. The results show that these microstructural changes affected the pitting corrosion resistance, as detected by ASTM G-61 and ASTM G-150 tests.

Austenitic stainless steels are high corrosion resistant alloys widely used in many industrial fields. Among this family of steels, AISI 317L stands out due to its higher localized corrosion resistance when compared to the traditional grades AISI 304L and AISI 316L. In some applications in oil refineries, the AISI 317L is being specified for services at moderately high temperatures. At the same time as it is sought to use new stainless steels, is also desirable to apply and develop emerging welding processes, replacing conventional ones, in order to achieve better behavior in service. In this respect, this work studied the effects of thermal aging on the toughness and resistance to pitting corrosion of AISI 317L steel weld metals produced by the Gas Tungsten Arc Welding (GTAW) and Friction Stir Welding (FSW) joining processes. After prolonged exposures at 450°C, for 200h, 300h and 400h, the microstructural characterization by scanning electron microscopy (SEM), toughness evaluation and anodic polarization tests in 3.5% NaCl solution were performed. The results showed that the increase of the exposure time in both weld metals caused a toughness decrease. The pitting potentials measured in the polarization tests also decreased with the aging at 450ºC.

The AISI 317L grade is an austenitic stainless steel with high Mo content (3.0 wt% min.). Due to the higher pitting resistance, this grade has replaced AISI 316L steel in many applications where the corrosion resistance is a critical property. However, the high Mo can induce phase transformations in high temperature services. In modern oil refinaries and petrochemical industries AISI 317L has been selected for temperatures as high as 550 °C. The goal of this work was to analyze the microstructural evolution and corrosion resistance of base and weld metal of AISI 317L stainless steel. The welded joints were produced by friction stir welding (FSW). The effect of prolonged exposure at 550 °C was investigated in specimens aged for 200h, 300h and 400h. After each aging treatment microstructural characterization was performed by scanning electron microscopy (SEM). Double loop electrochemical polarization reactivation tests (DL-EPR) were performed to evaluate the degree of sensitization of the samples. The results indicated that the increase of the exposure time at 550 °C promotes the formation of intermetallic phases, which causes corrosion decay of the weld metal.

A influência da precipitação de fases deletérias na resistência à corrosão em dois aços inoxidáveis superduplex (AISD) UNS S32750 com diferentes tamanhos de grão foi investigada. Foram realizados tratamentos isotérmicos de modo a introduzir na microestrutura diversas quantidades de fases deletérias, tais como chi, sigma, austenita secundária e nitretos de cromo. Nestas condições foram efetuados ensaios de corrosão por polarização anódica cíclica. Estudos por microscopias ótica (MO) e eletrônica de varredura (MEV) foram realizados para caracterizar as fases precipitadas e a sua cinética de precipitação. Os resultados permitem correlacionar os valores obtidos pelos ensaios de polarização com o percentual de fases deletérias precipitadas para diferentes condições de tratamento em AISD com composições similares, mas com diferentes tamanhos de grão. Observou-se ainda que o tamanho de grão e a temperatura de envelhecimento influem na cinética de precipitação, na morfologia e no tamanho das fases deletérias precipitadas, incidindo na resistência à corrosão por pites dos AISD.

The influence of deleterious phases precipitation in corrosion resistance was investigated in two superduplex stainless steel (SDSS) UNS S32750 with different grain sizes. Isothermal treatments were performed in order to introduce various quantities of deleterious phases, such as chi, sigma, secondary austenite and chromium nitrides. Under these conditions corrosion tests were carried out by cyclic anodic polarization method. Studies by light optical microscopy (LOM) and scanning electron microscopy (SEM) were performed to characterize the precipitated phases and the precipitation kinetics. The results allow to correlate the values obtained by polarization tests with the percentage of deleterious phases precipitates to the different conditions of treatments of SDSS with similar compositions, but with different grain sizes. It was observed that the grain sizes and aging temperature influence the kinetics of precipitation, morphology and size of deleterious phases. These features affect the corrosion resistance of SDSS.