ZnCr and ZnNb Cold Spray Coatings (original) (raw)

Abstract

Novel cold spray zinc-based alloy coatings have been investigated for corrosion protection of API 5L low carbon steel pipelines used in transporting natural gas. These different zinc alloy coatings are designed for use to help prevent internal corrosion and improve the lifespan of steel pipelines via sacrificial anodic protection. The corrosion behavior of these cold deposited zinc alloy coatings was investigated in CO2 saturated 3.5 wt.% NaCl solutions under simulated natural gas pipeline conditions. Electrochemical tests were conducted at 25 and 40 °C to determine the polarization resistance and galvanic current between each coating and the steel substrate in the galvanic couple. Tests were conducted at atmospheric, 1 bar, pressure. The results indicate that use of these coatings has the potential to increase the lifespan of steel pipelines above what a zinc coating alone could provide. Introduction In the US, natural gas accounts for one third of the energy consumed [1]. The gas pipeline network consists of more than 210 individual pipeline systems and nearly 300,000 miles, Figure 1, which makes it the most the most extensive pipeline delivery system in the world [2] [3]. The majority of the line pipe in current service was installed in the 1950's and 1960's. These pipelines are made of high grade carbon steel regulated by the American Petroleum Institute (API) [3] [4]. Failure of this pipeline can cause disruption in service and gas release into the environment. Corrosion is a primary failure concern, as carbon steel does not resist corrosion particularly well. Developing inexpensive, practical methods to increase the life of new and existing pipelines is of significant importance. Natural gas pipelines are usually buried underground, which puts them in contact with potentially corrosive soils. Currently coatings and impressed current systems are used to protect the external pipe from corroding [5]. The impressed current method works by supplying a remote external anode with a direct current, with the soil acting as the electrolyte. However, this only protects the external surface of the pipeline. Natural gas consists primarily of methane (CH4); some hydrocarbons, water; and gases, such as carbon dioxide (CO2); and dissolved ions, such as Cl− [3] [6]. While designed to be dry, usually less than 7 lbs. of water per MMCF of gas, it is not unusual to find liquid water. It has been shown when CO2 dissolves in water it forms a corrosive solution that can cause severe internal corrosion [7]. When transporting natural gas that contains corrosive species, the internal pipe has neither an anode nor an abundant electrolyte on which an impressed current can be utilized, but rather pockets of localized areas of attack.

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