Elsevier

Surface and Coatings Technology

Volume 257, 25 October 2014, Pages 182-205
Surface and Coatings Technology

Corrosion protection with hard coatings on steel: Past approaches and current research efforts

https://doi.org/10.1016/j.surfcoat.2014.08.069Get rights and content

Highlights

  • The corrosion behavior of hard coatings deposited on steel substrates is reviewed.

  • Micro- and macro-scale defects govern mainly the corrosion behavior of hard coatings on steel.

  • The coating microstructure seems to play only a secondary role in the corrosion behavior.

  • High corrosion resistance can be obtained for insulating or sacrificial coatings on steel.

  • Highest corrosion resistance seems to be delivered by a proper duplex treatment of steel.

Abstract

This work gives a review on published results on the corrosion behavior of hard coatings deposited on steel substrates by mainly physical vapor deposition (PVD) or in some cases by plasma-assisted chemical vapor deposition (PACVD). Corrosion investigations of our own lab have been added at several sites to complete missing knowledge or to introduce new non-published data. A detailed discussion of thin film growth, microstructure and the kind of defects inherent in hard coatings has been given. Then the influence of coating thickness and some processing parameters on the corrosion resistance of coated steel is demonstrated. After this incipient part, the corrosion behavior of hard nitride coatings, multilayer coatings, alloyed or modified nitride coatings, insulating nitride, oxide or oxynitride coatings and duplex treated steel is being presented. Own investigations and published results show that single layered, conducting nitride coatings can protect steel to some extent only if very thick coatings (≥ 10 μm) are applied or if energetic deposition methods (e.g. ion-assisted deposition) are used. Distinct improvements of the corrosion behavior of hard coatings can be obtained by a proper multilayer structure deposited by deposition methods allowing energetic condensation, i.e. possessing a high ion-to-atom flux ratio. Alloying TiN by a sacrificial element, like magnesium, is also capable of significantly improving the corrosion resistance of low alloyed steel. Amazing improvements have also been reported for insulating coatings like AlN, oxynitride coatings or oxide coatings. However, the most significant improvement seems to be the application of duplex treatments. Plasma nitriding + PVD coating or PVD coating + atomic layer deposition (ALD) prolonged the time to red rust in neutral salt spray test for > 2 days up to one week, respectively. The only disadvantage of the duplex treatments is the higher cost. Recently, mapping by a confocal microscope of TiN coating surfaces before and after salt spray testing showed that open porosities in micro- and macro-scale growth defects mainly govern the corrosion behavior of hard coatings on steel. The coating microstructure – i.e. the submicron-scale morphology – seems to play only a secondary role in the corrosion behavior.

Introduction

Corrosion protection is probably the largest area of coating application both in terms of total surface areas coated and business volume. The actual costs for damage caused by corrosion are estimated to be about 3–4% of the gross domestic product (GDP) per year of a country [1]. A simple calculation of the corrosion costs based on the GDP figures give enormously high numbers. Thus, the corrosion costs in 2013 for Germany are estimated to be about $103,000–$137,000 million, for the EU about $502,000–$699,000 million and for the whole world about $2,151,000–$2,868,000 million (estimated for 2012) [2]. Hence, improving the corrosion resistance of corrodible parts by proper surface coatings and surface modifications will help the society to save a lot of money and to preserve resources.

Corrosion protection requires a suitable coating material and a suitable coating structure. Additionally, an appropriate surface finishing – like a mechanical pre-treatment or a thermochemical treatment – can be necessary to prepare the surface of the parts to be protected in advance of coating application. The coating material must be either cathodically protecting or inert and defect-free.

Most hard coatings used for wear protection applications possess also a high corrosion resistance. However, when deposited on less noble materials like steel, brass, Al or Mg alloys, the coated parts, when exposed to a corrosive atmosphere, suffer from serious corrosive attack due to inherent coating defects or inhomogeneities (cracks, pores, transient grain boundaries) [3], [4], [5], [6], [7]. They open possible channels for the corrosive media to reach the substrate. In the case of a less noble substrate material galvanic corrosion at the substrate will occur. This kind of corrosion is localized to the defect area and is characterized by the anodic dissolution of the substrate material with a high anodic current density at the defect site. It is called galvanic corrosion and in the case of pores and pinholes as defects it is called pitting corrosion. The intensity of the corrosive attack strongly depends on the difference of the electrochemical potential between the coating and the substrate material in the respective electrolyte. If this difference is large enough, galvanic corrosion will occur. Though, other factors reducing the corrosive attack have to be taken into account, e.g. a) preventing a lowering of the pH value in the pit during the anodic dissolution process, b) the inhibition of the anodic dissolution by insulating or semiconducting layers or c) blocking of pinholes by corrosion products. Several authors [8], [9], [10] reported that the anodic dissolution of nitrided steel can release nitrogen into the electrolyte, which reacts with atomic hydrogen to form ammonium (NH4+). The amount of ammonium ions was only measured by Osozawa and Okato in Ref. [8]. The consumption of protons by nitrogen will prevent a lowering of the pH value or even increase the pH value to higher numbers. Lavigne et al. [11] also detected by X-ray photoelectron spectroscopy (XPS) the occurrence of NH3 and NH4+, but in the passive layer of Cr2N and CrN coated glass substrates after corrosion tests in aqueous 0.07 M Na2SO4 solution (pH 4). They concluded that the NH3/NH4+ species increase the interfacial pH, decreasing the driving forces for material oxidation. The influence of insulating or semiconducting layers will be discussed in more detail in 6.2 TiN/CrN multilayer coatings, 8 Coatings with high electrical resistivity. The blocking of pinholes by corrosion products was discussed for example by Schönjahn et al. and Hübler et al. [12], [13].

If the coating material does not cathodically protect the steel substrate, the corrosion behavior depends on the number and size of pores and/or on the chemical composition of incorporated particles (e.g. droplets [14] or foreign particles). These kinds of defects strongly depend on the deposition technique used for coating application. Vapor phase thin film growth is performed via production of species in a deposition source, transport of species in the gas phase and condensing the species on a substrate. The produced species can be atoms, molecules, ions and/or particles so-called droplets, which are generated during an arc on the deposition source (accidentally on magnetron cathodes or as part of the process on arc cathodes). These droplets will also be implemented in the growing film if they are not eliminated by a filtering procedure [15]. Thin films prepared by PVD/PACVD show a wide range of microstructures and properties, both of which are highly dependent on the preparation conditions. This has led to the development of structure zone models (SZMs) [16], [17], [18] also called structure zone diagrams (SZDs) [19], which relates the microstructure of a thin film to the reduced (or homologous) temperature TS/Tm (TS: substrate temperature, Tm: melting temperature of the coating material), total pressure and the kinetic energy of the condensing species, as a means of classifying the microstructures produced under different conditions. However, co-deposited active impurities can not only inhibit but also promote the structure evolution in vapor deposited polycrystalline thin films [18]. According to the SZMs/SZDs, microstructural development is in turn controlled by shadowing effects (zone 1), surface diffusion (zone T and zone 2), and bulk diffusion (zone 3) as TS/Tm increases [17].

Generally, dense films with low porosity are observed for higher substrate temperatures (TS) and/or for deposition methods providing sufficient kinetic energy to the condensing species The process itself is also called “energetic condensation” [20] or “energetic deposition”. As a high substrate temperature cannot be applied to temperature sensitive materials, energetic deposition can replace to some extent high substrate temperatures and lead to dense coatings. A number of deposition methods exist for energetic deposition of thin films [21], [22], [23], [24], [25], [26]: a) cathodic arc evaporation (CAE), b) pulsed laser deposition (PLD), c) ion beam assisted deposition (IBAD), d) inductive coupled plasma assisted magnetron sputtering (ICP-MS), e) electron cyclotron resonance magnetron sputtering (ECR-MS), f) hollow cathode magnetron (HCM) sputtering discharges or g) high power pulsed magnetron sputtering (HPPMS, sometimes abbreviated by HiPIMS or MPP).

The benefit of these energetic deposition methods can be exemplified by a publication from Hübler et al. [27]. They studied the dependence of the corrosion behavior of ion-beam-assisted (IBAD) deposition of TiN coatings on CK45 steel (heat treatable steel, material no. 1.1191, AISI 1045 or C45E) on the ion beam impact angle. They observed that ion-beam modification (Ar ion energy: 12 keV) of the TiN layer leads to loss of the columnar structure and improves the corrosion resistance of the coating. The loss of columnar structure was found at higher argon ion impact angles (40 and 55° off normal incidence). The authors revealed that the corrosion behavior of the IBAD-TiN samples was comparable to multilayered TiN samples (magnetron sputtering or IBAD).

The deposition of dense coatings seems to be an important issue in corrosion protection. However, some publications state that macroscopic defects, i.e. clearly visible coating defects, are the primary initiation sites for corrosion [28], [29]. Panjan et al. gave a classification of the defects and surface morphology found in hard coatings [30], according to their size:

  • Nano-scale: the morphology is determined by structural growth defects

  • Submicron-scale: the morphology is determined by nucleation mechanisms. Depending on growth conditions coarse columnar, fine columnar or amorphous growth modes are possible. The coating surface morphology is determined by the top morphology of columns.

  • Micro-scale: morphological features are determined by growth defects (voids, flake, over coated particles, pin-holes)

  • Macro-scale: the morphology is determined by the topography of bare substrate (scratches, pits, grooves, ridges).

Hence with energetic deposition mainly nano-scale and submicron-scale defects will be diminished. To some extent also macro-scale defects will be reduced by conformal thin film growth. However, we assume that the micro-scale growth defects like flakes and over-coated particles will pose the major challenge for energetic deposition to generate coatings without open paths down to the substrate surface.

Panjan et al. found that the average concentration of defects on various substrates and for different rotation modes in a commercial magnetron sputtering batch coater is comparable, while the scattering of the results is very high not only for the samples of different batches but also for the samples of the same batch. They concluded that different sources of dust particles must exist inside the vacuum chamber during the deposition process. The influence of the fixturing devices for substrates and components as well as possible formation of dust particles in the plasma have been considered as possible sources [31]. The delamination of small particles from chamber components (shields, fixtures) is much more frequent with the deposition time, because the compressive stresses in the deposit increases linearly with deposition time [30]. Therefore Panjan et al. concluded that a careful cleaning of the substrates, as well as frequent cleaning of vacuum chamber and fixture components, helps to reduce the defect density [32].

In the present contribution, the state of the art in the field of corrosion protection with hard coatings is presented. The results are discussed regarding the coating microstructure, defects, coating thickness and process parameters (deposition method). Options for improvement of the corrosion resistance – like multilayer structures, alloying or modification of nitride coatings, increasing the electrical resistivity and duplex treatments – will be reported, analyzed and evaluated. Finally the concluding remarks will also give an outlook to promising future investigations in the field of corrosion protection with hard coatings.

Section snippets

Corrosion measurement techniques for thin films

For evaluation of the corrosion behavior of film/substrate systems, three main kinds of technique can be found in the literature [33]:

  • Exposure of samples to a corrosive medium (e.g. in a salt spray test) and detection of the development of defects or corrosion products by visible inspection.

  • Quantitative dissolution measurements: The weight loss of the material into the corrosive solution is determined. However this method is not suitable for localized corrosion and should be only applied to

Microstructure and defects of hard coatings

As introduced in the first section the hard coatings have to be cathodically protecting or inert and defect-free if the coatings will be applied as corrosion protecting coatings. Since hard coatings are generally not cathodically protecting, the coatings have to be inert and defect-free. Consequently, the number of defects in the coatings has to be reduced to enhance the corrosion resistance of hard coatings. Therefore, this section will give a short overview on thin film growth morphologies

Influence of coating thickness

The thickness of hard coatings deposited on steel substrates has a significant influence on the corrosion behavior. This will be exemplified with the results of three publications.

In one of their papers on corrosion of nitride coatings, Barshilia et al. [49] also studied the influence of the coating thickness of TiN single and TiN/NbN multilayer coatings deposited on tool steel substrates using a reactive DC magnetron sputtering process. To ensure that only the inherent corrosion properties of

Different hard nitride coatings

The aim of this section is to compare the corrosion behavior of different nitride coatings. Wear resistant coatings like TiN, TiBN, CrN and Nb2N were deposited onto HSS by our own research group [7]. The coatings have been deposited by reactive magnetron sputtering in Ar/N2 atmosphere. Details of the process parameters can be found in Table 1. A dense coating surface morphology was observed for all four nitride coatings by SEM. The density of holes and growth defects was low and has been

Multilayer coatings

The deposition of multilayer coatings compared to single layer coatings has some advantages regarding the corrosion protection of steel substrates. It is assumed that multilayered coatings will have considerably lower porosity since the open structures reaching from the surface to the steel substrate might be interrupted by the intermediate films in the multilayer. By using multilayered coatings, it is also possible to relieve interface stresses if couples that develop tensile and compressive

Modification of nitride coatings

The corrosion behavior of hard nitride coatings can be influenced by a subsequent metal ion implantation which results in a change of the chemical composition of the top layer and to a closure of pinholes due to the ion bombardment. Furthermore, a less noble element like magnesium can be alloyed to TiN, acting as a sacrificial coating. For the alloying of aluminum to TiN some inconsistencies emerged with respect to the corrosion behavior of this ternary nitride coating. These three methods of

Coatings with high electrical resistivity

As outlined in the Introduction, corrosion of hard coatings on steel parts most often is electrochemical in nature. Generally the hard coating is nobler and the steel is less noble, forming a galvanic couple at defect sites in the presence of a liquid medium. For nitride-coated steels in a NaCl-containing solution, the iron (Fe2 +) is anodically dissolved at the defects (pores) and the electrons (originating from the dissolved Fe2 +) are traveling through the nitride coating to the coating

Duplex processes

Duplex processes (duplex treatments or duplex surface engineering) involve the sequential application of two surface technologies to produce a surface composite with combined properties which are unobtainable through the individual surface technology [93]. In the following two sections we will show the enhancement of corrosion resistance of steels by duplex treatments. The first duplex treatment will consist of plasma nitriding plus deposition of a hard PVD coating. The second one deals with

Conclusions

A thorough review on results of the corrosion behavior of hard coatings deposited on steel substrates has been presented. Nevertheless the authors are aware that this contribution cannot be complete and that the selection and significance of the presented topics is subjected to the authors experience and assessment.

As already reported by Jehn [51], it has been found that the corrosion studies are difficult to compare because of the different test conditions. Very often no correlation of the

Acknowledgments

The authors would like to thank the following colleagues: R. Bretzler, A. Heiß, B. Schöne (FIB/SEM investigations), K. Baumgärtner, L. Schmalz (corrosion testing).

The German Research Foundation (DFG) and the European Community (EC) are gratefully acknowledged for their financial support under contract no. DFG-Je-147/10, DFG-Fe-613/1, DFG-Fe-613/3 and NMP3-CT-2003-505948 (EC project acronym “HARDECOAT”) of part of the work.

The authors wish to commemorate Prof. Dr. Erich Bergmann (Switzerland)

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