Ind. Eng. Chem. Res. 2001, 40, 3-14
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Surface Treatments and Coatings for Metals. A General Overview. 1. Surface Treatments, Surface Preparation, and the Nature of Coatings Elisabete Almeida† INETI Surface Treatments and Coatings Laboratory, INETI, Estrada do Pac¸ o do Lumiar, 1649-038, Lisboa, Portugal
Given the diversity and complexity of technologies involved in the surface treatments and coatings sector, it is very difficult to find references in the existing literature that interlink most of them. The work that is presented here is aimed at filling that gap. In view of its length, it has been necessary to divide it into two parts. This first part deals with surface treatments and refers to the preparation of surfaces and the nature of coatings as two of the factors that affect anticorrosive protection by coatings. In the second part, the latter will be completed by consideration of coating application processes and environmental conditions during application and drying. Finally, a brief review will be made of new tendencies in the sector. 1. Introduction With industrial development, and the consequent need to use more sophisticated materials and equipment, industrialists face increasing difficulties about how to implement the surface characteristics to be used, how to obtain the unique properties of these surfaces, and how to reduce the cost of obtaining them.1 The variety and specificness of requirements demanded for the countless surface to be used make it impossible to rely on a small number of techniques, and in each case it is necessary to know which ones will be the most appropriate. It is relatively easy for a specialist to suggest or recommend the use of a certain metal to resist exposure in a certain corrosive medium. However, in terms of either manufacturing or availability, serious problems can arise for the large-scale obtainment and use of corrosion-resistant materials. For reasons fundamentally of an economic nature, steel continues to be the main metal used in construction despite its poor corrosion resistance. The economic reasons behind this are related to its ease of manufacture and its availability in a great variety of shapes and dimensions. Thus, in industrial reality, the vast majority of surface treatment and coating applications are destined to protect and/or improve the properties of ferrous surfaces. Today the technical-scientific area of surface treatments and coatings covers a vast scope, embracing different subareas such as mechanical, chemical, and physical-chemical surface treatments, thermal treatments, diffusion treatments, and ion bombardment, along with a wide range of traditional and advanced surface coating technologies. Given the vastness of this area, it is currently impossible to find a systematic classification of technologies which satisfactorily covers all of its scope without overlaps or repetitions.2 The present work approaches this technical-scientific area by focusing specifically on the aspects most connected with increasing the corrosion resistance of treated † Phone: 351 21 7165141. Fax: 351 21 7160901. E-mail:
[email protected].
and/or coated surfaces. It is dedicated to general metallic structures and equipment and not to special sectors such as automotive, aircraft, or appliance coatings. In the great majority of cases, anticorrosive protection by coatings is obtained by interposing resistance to the passage of the corrosion current. Coatings clearly offer greater resistance than the most habitual surface treatments, and the most common solution consists of the association of one or more surface treatments followed by the application of a coating. In the context of this work, a surface treatment is considered to be that which leads to a modification of the surface, either by modifying the material to a small depth (phase transition, interdiffusion, or ion implantation) or by modifying the surface itself (morphology, structure, chemistry, or others), always requiring the active intervention of the surface to be treated. Coatings are obtained by deposition of a foreign material on the surface to be treated, and in all cases a factor of greatest importance is the initial state of the metallic surface. 2. Surface Treatments The main surface treatments of industrial interest include thermal treatments, diffusion treatments, ion bombardment, mechanical treatments, and chemical and physical-chemical treatments. Though the last two groups are, from the viewpoint of anticorrosive properties, those of greatest interest for industrial application, brief reference is, nevertheless, made to the other groups because the fact that they modify surface characteristics means that they are sometimes related with the field of corrosion protection. 2.1. Thermal Treatments. Thermal treatments, as their name indicates, are surface treatments that fundamentally use temperature as the modifying agent to achieve significant structural modifications that improve the initial characteristics of the treated surfaces.3 For instance, thermal quenching treatments for tool steel may lead, among other transformations, to pearlitic, bainitic, or martensitic microstructures; in this particular case, the martensitic transformation is the most important. The total transformation of austenite
10.1021/ie000209l CCC: $20.00 © 2001 American Chemical Society Published on Web 12/02/2000
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Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001
Figure 1. Schematic representation of diffusion treatments.
Figure 2. Treatment of materials and surfaces by ion bombardment.
into martensite will correspond in this case to a maximum hardening of the structure. The heating of surfaces can be achieved by traditional methods or by others such as those involving the use of electron beams or lasers. Initially, the main application of laser thermal treatments was that of surface hardening in restricted areas of the surface to be treated, to locally improve its wear resistance. At the present time lasers are generically used to alter the mechanical, metallurgical, and corrosion resistance properties of the surface of materials. The main advantages of lasers, besides their application in delimited areas, are the absence of distortion (because there is no mechanical contact between the treated surface and the tool) and their high productivity, given the high degree of process automation. In practice, the use of lasers in thermal treatments permits increases in hardness and in friction, wear, corrosion, and fatigue resistance, as well as the treatment of singular geometries.4 Flame cleaning of metallic surfaces can also be considered a thermal treatment, albeit rather primitive, because it is based on the heating of the surface to be cleaned and the detachment of corrosion products and millscale, taking advantage of the different expansion coefficients of the interface components. Nowadays, the cleaning of works of art is being done with laser. 2.2. Diffusion Treatments. Treatments that involve the use of high temperatures and special environments can modify the composition of the surface upon which they act. As can be seen in the scheme shown in Figure 1, diffusion processes are named according to the nature of the chemical medium used in the treatment chamber, with the most frequent being carburizing (also known as cementation), nitriding, and carbonitriding. Furthermore, as can also be seen in the same figure, it is possible to use different treatment media such as gases,
liquids, vacuum, and plasmas. The properties of the treated surfaces depend significantly on the couple formed by the surface/specific treatment conditions. Under the heading of “other” diffusion coatings, “sherardizing” has proven to be of some interest in the anticorrosive protection of small manufactured steel components (screws, nuts, washers, etc.). This treatment, whose name is due to its inventor, consists of zinc diffusion on steel surfaces by the action of temperature and by compression in a rotary drum. Other treatments of this type are also known in which the diffused elements are chromium and others. 2.3. Ion Bombardment. According to Hantzergue,5 ion bombardment surface treatments not only modify the surfaces of materials but can also modify their outermost layers, according to the scheme shown in Figure 2. This classification of the scope of treatment is obviously related with the depth under the treated surface that the treatment can reach. Consequently, this type of surface treatment can physically-chemically modify the surfaces of materials by the production of energetic ions (cold plasma, ion gun, implanters, etc.), and ion-surface interaction can modify materials on the surface (by phase transition, ion interdiffusion, or ion implantation), and can even modify surfaces, either morphologically (polishing) or chemically (oxidation, nitriding, etc.), either by modification of their properties (electrical, optical, thermal, tribological, and chemical) or by ion erosion (ion cleaning). Despite their great interest for an extremely varied range of industrial applications, these types of surface treatments are not applied on large metallic equipment and structures because of the limited dimensions of the treatment equipment and its high cost. Among these techniques, special mention is made of ion implantation,6,7 which permits the controlled introduction of any element in a given material, irrespective of the affinity
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between them. The ions of the foreign element to be introduced combine with the atoms of the material to form an alloy to a predetermined depth next to the surface, up to a thickness of several tens of angstroms (1 Å ) 10-8 cm). This technique, initially used in the doping of semiconductors, with obvious advantages, when compared with classic techniques based on the diffusion of dopants in silicon at high temperatures, has in recent years been successfully introduced in the study of property alterations in other materials, such as metals and insulators, like glass and ceramics.8 2.4. Mechanical Treatments. As their name indicates, these types of surface treatments include all those that essentially act by causing the intervention of actions of a mechanical nature on the surface to be treated. This heading includes treatments such as shotpeening, with the aim of hardening surfaces and releasing residual stresses,9 and all blast-cleaning processes for metallic surfaces prior to coating application. As is mentioned more specifically in 3.1.1, such surfaces can only present good anticorrosive protection properties if they are suitably prepared before the application of the coating. In the field of industrial applications involving the use of a hot-rolled steel sheet (metallomechanics, shipbuilding, bridge building, etc.), mechanical treatments are preferred for the descaling of surfaces. Mechanical descaling to remove oxides, millscale, and other corrosion products can be achieved: (a) by the use of hand tools (scrapers, metallic brushes, etc.) or pneumatic or electric tools (rotary disks, brushes, chipping hammers, needle scalers, etc.) or (b) by abrasive blast cleaning.10 In the latter case, use can be made of metallic or nonmetallic abrasives, natural or manufactured. Shot and grit metallic particles are the most important of the metallic abrasives, while the nonmetallic abrasives include sands, granadas, zircons, melted metal slags, agricultural products, and others. Abrasive selection depends fundamentally on the specific application in question and on economic considerations. Abrasive blast cleaning can be carried out using special guns or throwing wheels, in manual, semiautomatic, or automatic installations. Mechanical descaling is of such importance in the field of anticorrosive protection that there are currently hundreds of standards and specifications (quality criteria) covering aspects such as the characteristics of the different abrasives (there is an international commission, ISO, that deals with this matter), blast-cleaning operations,9,10 and, very specially, the surface preparation grades obtained by means of the various techniques.10 Nowadays, anticorrosive coating specifications must almost obligatorily include the grade of surface preparation to be attained prior to application of the coating. 2.5. Chemical and Physical-Chemical Treatments. This surface treatments either removes material from the surfaces to be treated or alters their chemical composition. Both of these effects are of great importance in the specific field of anticorrosive protection. This is because treatments that remove material, of either organic nature (degreasing) or inorganic nature (chemical pickling), permit the suitable preparation of metallic surfaces to receive subsequent coatings.11 Furthermore, those treatments that alter the chemical composition of the surface in a controlled way can give rise to conversion films that are protective in them-
selves, as is the case of the anodizing of aluminum,12 or conversion films on metallic surfaces that contribute to increasing coating adhesion (as is the case of phosphating13 and passivation14). In particular, the relation with anticorrosive protection, cleaning process, or cycle is defined as the operation or set of operations that lead to the removal of contaminants from the surface to be treated, according to the intended application in each case.9 Among the degreasing processes integrated in manufacturing, mention can be made of (a) manual cleaning with solvents, (b) vapor-phase solvent cleaning, (c) ultrasonic cleaning, (d) flame cleaning, (e) corona-discharge cleaning, and (f) aqueous-phase immersion cleaning. In the field, use is made of processes such as (a) pressurized steam cleaning, (b) hot pressurized detergent cleaning, (c) foam cleaning, and (d) brushing cleaning. In the case of special applications, use is made of the process known as electrolytic polishing, in which the action of the chemical bath is associated with the action of an electric current, with the goal of obtaining the cleaning grade. With regard to the cleaning grade, clean surfaces can be classified in two categories: (a) atomically clean surfaces and (b) technologically clean or practically clean surfaces. The former are demanded in special processes and can only be obtained in a high vacuum (
