Dry thermomechanical stripping by cryogenic nitrogen jet

Pickling involves removing a layer of material (coating or oxide layer) present on the surface of a substrate. The oldest techniques are mechanical (sandblasting, shotblasting, waterjet), chemical and thermal at high temperatures (laser, induction) or low temperatures (CO ₂ dry ice, dry ice, cryogenic CO ₂ ). In this paper, a recent and emerging technology based on cryogenic N ₂ nitrogen jetting close to the supercritical state, proceeding by dry process and combining mechanical effects (jet impact energy – with or without abrasive –, blast effect) with low-temperature thermal effects is described. This new technology is revolutionizing paint stripping thanks to its efficiency, flexibility and versatility, and its very low environmental impact, thanks in particular to the natural recycling of nitrogen into the atmosphere after the stripping operation; what's more, it's safe to use for operators.

Cryogenic nitrogen stripping involves impacting the surface of a "coated" material with a jet of nitrogen that has been previously cooled (– 110 to – 160 °C) and pressurized (300 to 3,700 bar) to bring it to a state close to supercritical. At the nozzle outlet, these characteristics change rapidly during expansion in the atmosphere, but they are sufficient to degrade various types of coating by creating thermal shocks on impact, in the coating thickness, and at the substrate/coating interface. Thermal shock generates cracks in the coating or disbonds at the interface; the dynamic pressure of the jet and the blast effect ultimately lead to partial or total removal of the substrate. Substrate damage-free etching is controlled by the right choice of operating parameters, depending on the nature of the coating to be removed.

The fields of application are many and varied. The technique was initially developed for stripping materials in the aerospace and nuclear industries. Recent work has demonstrated its value for stripping foundry molds, metal paints and metal oxide coatings, and for fracturing porous materials such as concrete.

The advantage of this technology is that, after impact, the nitrogen, the main constituent of the ambient air, evaporates. Totally neutral for the environment, this technology eliminates the disadvantages of other mechanical or chemical stripping techniques, which require the treatment of effluents or secondary residues.

The aim of this article is to describe the new cryogenic nitrogen blasting technology and compare it with other traditional blasting processes. The techno-economic approach used will give the reader a precise view of this emerging technology, enabling...