Behavior of Monel 400 in Arabic Gulf seawater

Monel 400 is exposed to the different natural aerated seawater at the different times in the Arabian gulf. Monel is a solid solution alloy that can is hardenable by cold processing. It is widely used in the different applications such as chemical processing, gasoline, fresh water containers, crude petroleum stills, pumps, propeller shafts, seawater fixtures etc. The corrosion of alloy 400 reduces when more nickel content is added.

The natural sea water was collected from Arabian Gulf, Monel plate was used. Alloy 400 rods were used for electrochemical measurements to develop the coupons for use in the sea water. To evaluate the pitting corrosion of alloy in the salt solution, the SEM and EDX tests were performed. The presence of oxygen and carbon in the alloy result in the development of oxide layer and the pit is found to be covered with this layer in addition of other materials.

An enlarged space in the pit occurred on the surface of alloy that is exposed to free aerated stagnant salt solution for 160 days. It is found that the developed pit is deep and broad with corrosion materials stacked in few regions. The presence of chloride ions increases the potential difference across the layer. The regular development of voids result into localized damage of the passive layer that dissolves faster than other components.

The affected regions were found to be enriched of copper whilst the regions around the active sites have higher nickel concentration. The corrosion of alloy 400 occurs by the specific leaching of nickel from the alloy offering a spongy copper rich material in the pit base.

The chlorine and sulphur elements present in the marine water collect in the pit and interact with iron and nickel that are present in the alloy therefore iron and nickel are slightly present in pit as compare to their real availability in the Monel alloy.

With the passage of time, the layer dissolves because of pre-immersion of alloy’s electrode in the salt solution, resulting into pitting. Pits develop in the areas where the adsorbed oxygen on the alloy’s surface is replaced by the vigorous particles for example chloride ions that are found in the Arabian Gulf sea water. It is because the choride ions have smaller diameter that allow their entrance in the protective oxide layer and replaces oxygen at the places where the metal and oxygen are not strongly connected.

The development of corrosion materials nominally secures the alloy because the potential reduced in the positive direction. The reduction in the corrosion rate is made possible by the development corrosion materials as wells as the potential of the salt solution to develop layers on the surface of alloy.

So overall the Monel alloy 400 receives general and localized corrosion in the stagnant Gulf seawater. The pitting is noticed on the alloy caused by chloride ions and dissolution of nickel. Increased exposure reduces the current and increases security potential in the negative direction. The pitting increases with increasing the immersion time length in the sea water.