Korrosionsverhalten gespritzter Zink‐Aluminium‐Überzüge in Kurzzeitbewitterungstests

Corrosion behaviour of thermally sprayed zinc‐aluminium‐coatings in short‐term corrosion tests By alloying aluminium to zinc the corrosion resistance can be increased in a multiplicity of media. Thermally sprayed ZnAl 15‐coatings show a higher corrosion resistance than thermally sprayed zinc‐ or hot dip galvanized coatings. This paper presents the results of short term corrosion tests (salt spray test, Kesternich test) with sprayed ZnAl‐coatings with an aluminium content of 2 to 50 M.‐%. The investigations verify very clearly the influence of aluminium on the corrosion behaviour. Depending on the aluminium content the corrosion resistance shows a maximium with a medium content of about 15 or 22 M.‐%.     

The Corrosion Behavior of Cold Sprayed Zinc Coatings on Mild Steel Substrate

Zinc and its alloy coatings have been used extensively for the cathodic protection of steel. Zinc coating corrodes in preference to the steel substrate due to its negative corrosion potential. Numerous studies have been conducted on the corrosion behavior of zinc and its alloy coatings deposited using several techniques viz., hot dip galvanizing, electrodeposition, metalizing or thermal spray etc. Cold spray is an emerging low temperature variant of thermal spray family which enables deposition of thick, dense, and pure coatings at a rapid rate with an added advantage of on-site coating of steel structures. In the present study, the corrosion characteristics of cold sprayed zinc coatings have been investigated for the first time. In addition, the influence of heat treatment of zinc coating at a temperature of 150 °C on its corrosion behavior has also been addressed.     

Korrosionsverhalten von feuerverzinktem Stahl in erwärmten Wasser1

Corrosion behaviour of hot galvanized steel in warm water Pipe specimens made of commercial grade galvanized steel and zinc have been tested in comparison to pipe specimens having modified zinc coatings. The specimens were corroded in 11 test lines with warm water at approx. 60 °C and cold water with continuous and intermittent flow. Dortmund town water with polyphosphate and orthophosphate additions in individual test lines was used for the tests. One test line was operated with warm water from a water heater with cathodic vessel protection according to the Guldager method. The investigation included visual assessment, determination of mass loss, and electrochemical measurements. The main results were as follows: 1. The localised corrosion tendency is not determined by the amount of potential ennoblement, but rather by the inhibition of the cathode reaction on the surface layer formed, which can be read off the cathodic current density vs. potential curve. The inhibition of the cathode reaction can be affected both by variations of the material and by variation of the water quality. 2. The tested materials with modified coatings featured throughout a poorer corrosion behaviour in warm water than commercial zinc coatings according to DIN 2444. 3. The phosphates used for the investigation induce an inhibition of the cathode reaction in zinc. Zinc-iron alloy phases in warm water fail to ensure an appropriate effect. A treatment of the water according to the Guldager process ensures a strong inhibition of the cathode reaction both with zinc and especially with zinc-iron alloy phases and induces an important improvement of the corrosion behaviour of galvanized steel in warm water.     

Modelling the relationship between microstructure of Galfan-type coated steel and cut-edge corrosion resistance incorporating diffusion of multiple species

This paper describes a considerable extension to a previously documented [S.G.R. Brown, N.C. Barnard, 3D computer simulation of the influence of microstructure on the cut edge corrosion behaviour of a zinc aluminium alloy galvanized steel, Corrosion Science 48 (2006) 2291-2303], first-order model used to simulate the localized degradation experienced in Zn-4.5 wt% Al steel coatings exposed to 5% NaCl aqueous solution. The temporal localization and intensity of discrete corrosion effects are predicted using established relationships and, in contrast to earlier models, the evolution of multiple concentration fields is included and calculated using straight-forward finite difference techniques. Changes in composition are included in the quantification of both anodic and cathodic processes involved in the corrosion of steel coatings in contact with aerated saline solutions. Reported [J. Elvins, J.A. Spittle, D.A. Worsley, Microstructural changes in zinc aluminium alloy galvanising as a function of processing parameters and their influence on corrosion, Corrosion Science 47 (2005) 2740-2759] and modelled performances of typical Galfan composition coated steels are evaluated for different coating microstructures undergoing so-called cut-edge corrosion. In summary, this latest model successfully matches measured rates of metal loss during localized corrosion. Additionally, the inclusion of multiple species diffusion functionality has greatly improved the simulation of the cathodic reaction in particular and the overall form of the current density distribution near the corroding surface.     

Steel cathodic protection afforded by zinc, aluminium and zinc/aluminium alloy coatings in the atmosphere

Zinc has traditionally been the metallic material most widely used to protect steel against atmospheric corrosion due to its ability to afford cathodic protection to steel in all types of natural atmospheres. In recent decades, aluminium and zinc/aluminium alloy coatings have been used instead of zinc in certain atmospheric applications. Although these coatings present some advantages over zinc, they are not able to cathodically protect steel substrates in all types of natural atmospheres. The present paper assesses the cathodic protection afforded by Al (flame spraying), Al/13 Si (hot dipping), 55Al/Zn (hot dipping), Zn/15Al (flame spraying), Zn/5Al (hot dipping), Zn (hot dipping), Zn (discontinuous hot dipping) and Zn (electroplating). Aluminium and aluminium-rich alloy coatings (55%Al/Zn) provide cathodic protection to the steel substrate only in atmospheres that are highly contaminated with chloride ions (>100 mg Cl⁻ m⁻² day⁻¹) where these coatings become active.     

Beschichtungssysteme für Verbindungselemente in Kontakt mit Leichtmetallen

Coating systems for joining elements in contact with light metals Galvanic corrosion of light metal alloys can only be avoided, if the steel fasteners are given an electrochemically compatible protection or if the corrosion circuit is interrupted by electric insulating layers. Current density vs. potential curves of chromated zinc alloy coatings show, that by means of zinc nickel coatings a largely electrochemical adaption to the equilibrium potential of quenchaged aluminium alloys of the type AlMgSi is possible. On the other hand, the equilibrium potentials of chromated systems based on ZnFe, ZnCo respectively, are too low to avoid galvanic corrosion on aluminium. In practicerelated corrosion tests on joining elements, this result was confirmed. Besides this, zinc- and aluminium-bearing sintersystems, galvanic tin depositions as well as ternary mechanical coatings based on ZnSnAl have been proved to be compatible with AlMgSi-type alloys. For coating systems in contact with magnesium a satisfactory protection against galvanic corrosion can be achieved effectively with insulating top coats. The results with suitable duplex systems are presented and recommendations for practical applications are derived.     

Adhesion mechanisms of arc-sprayed zinc on concrete

Arc-sprayed zinc coatings can provide cathodic protection against corrosion to steel reinforcement in concrete. Because the adhesion of sprayed zinc on concrete is of major concern, the parameters related to zinc deposition and concrete preparation that affect the adhesion have been previously investigated. However, little attention has been devoted to determining which basic mechanisms are responsible for the adhesion of molten zinc on concrete. Because the interaction of molten zinc droplets with the concrete surface is considered physical, this paper is focused on the influence of surface patterns on the adhesion of arc-sprayed zinc coatings. Concrete surfaces were characterized by image analysis and profilometry techniques to ascertain which surface pattern or components could affect the adhesion of zinc. A modified root mean square (RMS) surface roughness was derived to take into account the different surface mor-phologies seen by sprayed zinc droplets. This modified RMS surface roughness was found to be directly related to the measured bond strength of arc-sprayed zinc on concrete. After the surface profile on con-crete is measured and the surface constituents are considered, the bond strength of arc-sprayed metals on concrete can be forecasted for given deposition parameters.     

Korrosionsschutzwirkung von Beschichtungen am Defekt – Einfluss der Oberflächenvorbereitung und der Pigmentierung der Grundbeschichtung

Corrosion‐protective effect of organic coatings at the defect – influence of surface preparation and anti‐corrosive pigments of base coating In practice of corrosion protection a clear influence of surface preparation and pigments of base coating has been widely observed on defect‐induced delamination of organic coatings. In the presented paper this mechanism was studied using the Scanning Kelvin‐Probe. This instrument enables determination of the corrosion potential by high resolution contactless measurements even with presence of insulating organic coatings. During these investigations the following results were obtained: Cathodic delamination takes place in case of coatings with zinc phosphate primers on blast cleaned steel substrates. Using zinc dust primers, especially the edges of the defect are cathodically protected by anodic dissolution of zinc as part of the zinc dust primer. Because of the formation of zinc oxides, increasing load duration can lead to a deactivation of zinc dust and a progression of the corrosion process below the coating. On hand‐cleaned steel surfaces rust reduction takes place at the edge of the defect in form of a cathodic partial reaction which is independent from pigments of the base coating. This reduction is increased by the galvanic cell. In case of cyclic loading reduced rust is reoxidized during dry phases and can re‐participate in the process during wetting. In the result the thickness of the rust‐layer is growing and phase transformations in the rust lead to the loss of adhesion of the coating starting from the edge of the defect     

超细锌粉无机硅酸锌涂层的结构及耐蚀性研究

制备了3种超细锌粉作填料的无机硅酸锌涂层,用FTIR、SEM对涂层的固化机理和结构进行了研究,对3.5%NaCl溶液浸泡腐蚀后的试样表面形貌进行了观察,通过电化学试验研究了涂层的耐蚀性.结果表明,无机硅酸锌涂层的结构为:Zn粉颗粒间以Si-O-Zn聚合物网络键合,涂层与底材钢基体间是以化学键结合的微观多孔的涂层结构;涂层的耐蚀性随Zn粉颗粒度的减小而显著提高.涂层的耐蚀机理主要是电化学阴极保护作用以及腐蚀产物填塞涂层孔隙的物理屏蔽作用.     

Corrosion resistance of cerium‐based chemical conversion coatings on AA5083 aluminium alloy

In this paper the effect of several parameters, such as temperature, time of immersion, cerium ions and hydrogen peroxide concentration, pH of the conversion solution, on the composition and morphology of the conversion layer are investigated as well as on its corrosion resistance in chloride environments. The cerium‐based chemical conversion coatings ennobles the corrosion potential and inhibits both the cathodic and anodic reactions rate. Using a cerium (III) chloride solution a not homogeneous coating is obtained and agglomerates with a “dry‐mud” morphology of mixed cerium‐aluminium oxide are deposited above the cathodic intermetallic particles, while using a cerium (III) nitrate solution the coating is more uniform but thinner than that obtained with cerium (III) chloride. Solution temperature below 50°C and time of immersion of 10 minutes produces a coating with better corrosion resistance.     

汽车表面达克罗防腐涂层的组织和性能

介绍了达克罗技术在汽车工业中的应用情况，对达克罗防腐涂层表面组织形貌和耐腐蚀性能进行了研究，并和镀锌、镀铝涂层的耐腐蚀性能进行对比。结果表明，锌、铝、铬等元素在达克罗涂层中分布均匀，片状的锌粉和铝粉层叠，而无定形的铬酸盐聚合物分布在锌、铝之间。涂层覆盖在被保护工件表面，形成牢固的防腐蚀保护涂层。     

Recent Developments in Metal Spraying by the Powder Process

General principles of metal spraying by the powder process are described, and an account is given of an improved powder-spraying pistol. In this apparatus, powder is conveyed into an oxy-gas flame by any desired conveying gas under pressure: this allows the application of denser, less oxidized and better adherent coatings of zinc, aluminium, etc., than does the older spraying pistol wherein the powder is sucked into the pistol in an air stream, and also the spraying operation is faster and more efficient.

A simple quantitative adhesion test for sprayed metal coatings is described.

Some recent specialized techniques and applications are outlined, including the production of sprayed coatings of very low and of very high resistivity, the spraying of steels, the production of “aluminized” coatings by powder spraying followed by heat treatment, the production of hard, non-porous surfaces by “flaming” special sprayed coatings, and the improvement of the adhesion of zinc and aluminium coatings to be had either by an undercoat of sprayed steel or by preheating the base metal.     

Degradation behaviour of ZrO2-Ni/Al gradient coatings in molten Zn

In order to improve the corrosion resistance of metallic materials in molten zinc, ZrO₂-Ni/Al gradient coatings were sprayed on the surface of the Fe-0.35-0.44 wt.% C steel. The corrosion behaviour and corrosion mechanism of the ZrO₂-Ni/Al gradient coatings in molten zinc were studied. The ZrO₂-Ni/Al gradient coatings on the surface of steels prolonged the lifetime of samples and changed the corrosion behaviour of the samples in molten zinc. The lifetime of the ZrO₂-Ni/Al gradient coatings immersed in molten zinc at 620 °C is 28 days, which is 4 times as long as that of the general ZrO₂ coatings. The ZrO₂-Ni/Al gradient coatings were corroded in molten zinc at 620 °C, which was caused by zinc atom diffusing along the crystal boundary and pores of the ZrO₂-Ni/Al gradient coatings, and reacting with Ni/Al particle in the ZrO₂-Ni/Al gradient coatings. The corrosion mechanism of the coatings in molten zinc at 620 °C was crystal boundary corrosion, pitting corrosion and reaction corrosion.     

High‐temperature corrosion behavior of some post‐plasma‐spraying‐gas‐nitrided metallic coatings on a Fe‐based superalloy

The objective of the present study is to propose a cost‐effective process for modifying commercially available coatings by gas nitriding using commonly available equipment and starting materials. Al–Cr and Ti–Al metallic coatings were deposited on Superfer 800H (Fe‐based superalloy) using a plasma spray process. Then the gas nitriding of the coatings was done in the lab and the parameters were optimized after conducting several trials on plasma‐sprayed‐coated specimens. Characterization and high‐temperature corrosion behavior of coatings after exposure to air and molten salt at 900°C were studied under cyclic conditions. Techniques like XRD, SEM/EDX, and X‐ray mapping analysis were used for the characterization of the coatings and analysis of the oxide scale. Both the coatings successfully protected the substrate and were effective in decreasing the corrosion rate when subjected to cyclic oxidation (Type‐I hot corrosion) at 900°C for 50 cycles in air and molten salt (a salt mixture of Na₂SO₄–60%V₂O₅ dissolved in distilled water). Based on the findings of the present study, the coatings under study are recommended for tapplications to super‐heater and reheater tubes of boilers and all those surfaces that face fireside corrosion, such as fluidized beds, industrial waste incinerators, internal combustion engines, gas turbines or steam turbines, to provide protection against degradation in these environments. The cost of the product/process is approximately Rs. 0.62 per mm² in case of Al–Cr coating and Rs. 1.86 per mm² in case of Ti–Al coating.     

Metal Coatings on Steel at Lighthouse Beach,Lagos

Abstract

Trials are reported of zinc, lead and aluminium coatings exposed for 8·7 years at Lighthouse Beach, Lagos. Sprayed aluminium coatings gave good results. Zinc coatings applied by hot-dipping or electrodeposition were also successful but, after 8·7 years, sprayed zinc coatings were beginning to fail. Electrodeposited lead coatingscould not be recommended. The thickness of coating applied to the steel specimens was only 50–80 μm.     

15 Jahre deutsche Korrosionforschung im Forschungsprogramm Korrosion der Deutschen Bundesrepublik — Teil II

15 Years of German corrosion research in the research programme corrosion of the Federal Republic of Germany — Part II The second part of this report (Pt. I se WuK 24 [1973] 6; 525–535) deals in particular with the possibilities of corrosion protection, and in particular inhibitors, electrochemical methods (cathodic protection), electrolytically deposited metal coatings (including the feasibility of influencing their properties by galvanizing conditons), sprayed and vacuum deposited coatings non-metallic inorganic coatings (passivating layers such as anodizing etc., as well as their preparation mechanisms) and organic coatings. A concluding part deals with test methods, on the one hand methods to test the corrosion behaviour of materials in various media and under various conditions on the other hand methods to test the efficiency of various corrosion protection methods. 434 ref.     

Development of a new combined corrosion protection system for chloride‐contaminated reinforced concrete structures

Chloride‐induced damage of reinforcing steel is especially for parking garages and bridges often very severe as large amounts of chlorides act on horizontal surfaces which could lead to fast ingress of these detrimental ions up to the level of the reinforcement and subsequently to high rates of corrosion. In order to avoid the disadvantages of conventional rehabilitation (unreliable prognosis with patch repair or high costs and regular maintenance with cathodic protection using impressed current) a new combined protection system was developed where the principle of drying out the concrete (by means of a surface protection system) is combined with a temporary cathodic protection (by means of a sprayed zinc layer) during the transitional period of the drying out process. This new system was tested both in the laboratory and on‐site at a pilot application. Based on the results obtained the possibilities and limitations of the new system are discussed in this paper.     

Technisch-ökonomische Auswertung der Korrosionsverluste an Stahlkonstruktionen im Gebiet Nordböhmen

Technico-economical evaluation of corrosion losses of steel structures in Northern Bohemia The corrosion rates of structural steel, zinc and aluminium are estimated for the region of Northern-Bohemia on the basis of several years' weathering in 15 atmospheric weathering-stations; in addition the useful life of three-layer paint systems was evaluated in factories and electricity distribution systems in the vicinity of such weathering stations. In this context the cost are compared for replacement of corroded steel parts (thickness reduction by 50%) and by the use of corrosion protection systems (hot-galvanizing, aluminium sprayed cating and three-layer paint system including repainting at defined intervals). Hot-dip galvanizing appears to be the optimum solution with respect to cost as well as the amount of work required.     

Zinc runoff from organically coated galvanised architectural steel

The metallic runoff from the exposed cut-edges of galvanised steel substrates (zinc and an experimental zinc/aluminium alloy) over coated with organic coatings have been quantified over a 30-month period of external exposure. The organic coatings comprised a 200 μm PVC plastisol coating on one side and a 15 μm polyester coating on the other with compatible chromate based pre-treatments and primers. A novel method of increasing cut-edge length per unit area has been developed in which samples are punched with a regular array of holes. This increases the available cut-edge length per unit area by three orders of magnitude over a standard roofing panel. The only measurable leachates detected (>0.5 ppb (part per billion)) over the exposure period have been zinc and iron. All the samples showed significant reduction in zinc runoff upon prolonged exposure with zinc leaching concentrations over a three-month sampling period typically falling from a maximum 1-2 ppm (part per million) upon initial exposure to 0.05-0.1 ppm after 12 months exposure in the harshest industrial and marine sites. Metal ion runoff has been lowest in rural weathering locations (typically 0.5 ppm in a three-month exposure period upon initial exposure falling to <0.05 ppm over the same time period after 12 months exposure).In general at the more severe marine and industrial sites the greatest metal runoff levels were detected from organically coated substrates made from experimental metallic coatings with a 4.95% aluminium, 0.05% magnesium and 95% zinc metallic coating with less runoff leaching from 99.85% zinc coatings. In the former substrate this is due to the presence of subsurface pro-eutectic zinc rich dendrites that are exposed at the cut-edge but are not present in the corrosion resistant aluminium rich surface of the material. In these cases zinc leaching is independent of whether the samples are over-coated with organic layers.     

多道富锌基涂层在NaCl溶液中的电化学行为研究

多道富锌基涂层的电化学行为研究表明,涂层中的Zn粉未表现出明显的阴极保护作用,即使在富锌涂层上覆盖几个微米厚的环氧清漆涂层, Zn的阴极保护作用也会变得非常微弱. Zn粉对钢铁的保护作用在于Zn粉腐蚀产物对涂层的“自修复能力”.在环氧清漆覆盖条件下,无机富锌涂层中Zn粉的反应活性比有机富锌涂层中Zn粉的高,即Zn粉的反应速度更快.