Corrosion–erosion of TiN-PVD coatings in collagen and cellulose meat casing

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.