Corrosion–erosion of TiN-PVD coatings in collagen and cellulose meat casing J. de Damborenea a , C. Navas a , J.A. García b, ⁎ , M.A. Arenas a , A. Conde a a Departamento de Corrosión y Protección, Centro Nacional de Investigaciones Metalúrgicas CENIM-CSIC, Av. Gregorio del Amo 8, E-28040 Madrid, Spain b Asociación de la Industria Navarra, San Cosme y San Damián s/n, E-31191 Pamplona, Spain Received 25 July 2006; accepted in revised form 2 October 2006 Available online 21 December 2006 Abstract The meat casing industry works with highly fibrous materials and low pH conditions. The pumps propelling the gel must therefore be highly corrosion–erosion resistant. In fact, the most common failure in this medium is corrosion, erosion or, most commonly, a combination of both. Coatings are thus required to protect the pump surface and maintain the operational conditions. TiN-PVD coatings seemed to be an attractive alternative amongst the range of options to extend pump lifetime. However, despite the greater chemical inertia expected of TiN, the combined effect of corrosion and erosion can induce a fast attack on the material. The present paper analyses failure of a TiN coating with a single-layer configuration in contact with an edible slurry of collagen used in the meat casing industry. The degradation mechanism comprises two different steps. Firstly, the acidic components reach the base steel through the pores, promoting severe dissolution of the steel substrate and leaving the TiN coating without support. Secondly, the coating detaches under working conditions and merges with the casing solution. Finally, the fibres together with the hard particles erode the pump walls, leading to fast deterioration. © 2006 Elsevier B.V. All rights reserved. Keywords: PVD-TiN; Wear; Meat casing; Corrosion 1. Introduction Traditionally, natural casings are extracted from the digestive tract of edible animals, primarily from the submucosa, a largely collagen layer of the intestine [1]. However, natural casing is more expensive than the artificial alternative, and hence is primarily used for the most expensive sausages and cold meats. Artificial meat casings can be manufactured from various raw materials [2], most of them rich in collagen and cellulose. Their main advantages are high resistance (natural casings are prone to break) and easy manufacturing (producing uniform size and shapes). Basically, the casing consists of collagen and cellulose fibres that form an acid slurry. Subsequently, this gel is extruded through a mixer and acquires integrity after chemical treatment. During the process, the casing is propelled by different lobe pumps exposed to a highly aggressive solution. The collagen and cellulose slurry is usually kept in gel form at pH between 1 and 3 prior to handling. The lobe pumps commonly used in the food industry are generally made from stainless steel (mainly AISI 316) because of its high corrosion resistance. However, the materials used in the meat casing industry are both highly fibrous and low pH, requiring high corrosion–erosion resistance in these pumps. In fact, the most important causes of damage to these pumps are corrosion, erosion in the form of abrasion or, most commonly, a combination of both. Physical vapour deposition (PVD) has aroused considerable great interest in recent years since it permits the deposition of dense, compact coatings, leading to improved chemical and mechanical properties. The technique permits the production of a wide range of hard coatings that are extensively used in manufacturing because of their high wear and chemical resistance, relatively low cost and easy coating procedure. Since the commercial success of TiC-coated hard metal tools in the late 1960s, the interest in hard coatings has steadily increased. In the class of hard coatings, which range from Surface & Coatings Technology 201 (2007) 5751 – 5757 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Asociación de la Industria Navarra, San Cosme y San Damián s/n, E-31191 Pamplona, Spain. E-mail address: jagarcia@ain.es (J.A. García). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.10.009