Vergleichende Untersuchungen an verzinkten Stahlrohren mit und ohne Elektrolyse-Schutzverfahren

Comparing tests on galvanized steel pipes with and without cathodic protection The corrosion behaviour of commercial graduated galvanized steel pipes installed into a warm water supply system with cathodic vessel protection is compared to the behaviour of pipes in a similar system without cathodic protection.     

Corrosion Inhibition of Galvanized Steel in NaCI Solution by Tolytriazole

Corrosion inhibition of tolytriazole for galvanized steel was studied in 5 mM NaCl by using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET). The results of EIS and polarization tests indicate that tolytriazole is effective in corrosion inhibition of galvanized steel. As the concentration of tolytriazole is increased to 0.01 M, the inhibiting efficiency reaches above 98%. The low values of anodic and cathodic current density in SVET maps suggest that the complex of tolytriazole with galvanized steel inhibits the anodic and cathodic reactions of corrosion of zinc. The adsorption behaviour of tolytriazole is found to conform to Langmuir adsorption isotherm, which is typical chemical adsorption.     

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.     

Über das Korrosionsverhalten von Zink und feuerverzinktem Stahl in erwärmtem Wasser

Corrosion behaviour of zinc and hot dip galvanized steel in warm water The corrosion rate of zinc and hot dip galvanized steel in continuously flowing tap water is but little influenced by water temperature. The slight decrease of the corrosion rate which has been found at higher temperatures (60°C) can be attributed to the preferential formation of zinc oxide at these temperatures, this oxide being less soluble than the hydroxide formed at lower temperatures. The potential shift toward more noble values at higher temperatures does, however, depend from water composition and may eventually start as early as at 35 °C; in certain critical potential ranges the uniform corrosion may even be changed into pitting. Addition of phosphate to the water may reduce either the corrosion rate or it may restrict the potential shift (in this latter case the appearance pitting is prevented). The measurement of the electrode potential (not of the polarization resistance) thus yields information on the danger of pitting corrosion. The quality of the zinc surface seems to be important in those cases where the ZnFe alloy layer protrudes to the zinc surface.     

Corrosion Inhibition of Galvanized Steel in NaCl Solution by Tolytriazole

Corrosion inhibition of tolytriazole for galvanized steel was studied in 5 mM NaCl by using potentiodynamic polarization and electrochemical impedance spectroscopy(EIS), scanning vibrating electrode technique(SVET). The results of EIS and polarization tests indicate that tolytriazole is effective in corrosion inhibition of galvanized steel. As the concentration of tolytriazole is increased to 0.01 M, the inhibiting efficiency reaches above 98%. The low values of anodie and cathodic current density in SVET maps suggest that the complex of tolytriazole with galvanized steel inhibits the anodic and cathodie reactions of corrosion of zinc. The adsorption behaviour of tolytriazole is found to conform to Langmuir adsorption isotherm, which is typical chemical adsorption.     

Case study: Analysis of coating failure on hinged steel boxes

A systematic failure analysis was performed on three large steel boxes in which the coating system had blistered, extensively delaminated, and subsequently corroded. These boxes were a sample from a larger batch of boxes that had been recalled due to severe corrosion.Despite the fact that limited information on the process history was provided, we were able to determine that two out of three boxes were neither galvanized nor protected by a zinc-rich primer to provide cathodic protection. The coating system comprised an epoxy primer, zinc-rich intermediate coat followed by an acrylate urethane top coat. A zinc-rich coating should never be applied over a nonmetallic coating because cathodic protection of the underlying steel can only take place if the zinc-rich coating is in direct electrical contact with clean steel.The third box was galvanized and primed with a urethane-alkyd primer followed by an acrylate urethane top coat. Alkyd-type coatings should not be applied over zinc (galvanizing) because the corrosion products of zinc are alkaline. Alkyd-modified coatings are very sensitive to alkalinity and a saponification reaction occurs at the zinc-alkyd interface. This degrades the alkyd and causes it to peel or delaminate from the zinc alloy used for the galvanization.The epoxy primer showed unusually severe air bubbles (or pockets) that could have been due to saponification and/or solvent entrapment. In either case, these large pockets weakened the bond between the primer and galvanized layer and allowed delamination to occur.Our finding that chlorides were present on the surface of the epoxy primer indicated that the boxes might have been exposed to a marine or coastal atmospheric environment. Marine or coastal atmospheric corrosion is generally considered to be one of the most severe atmospheric corrosion environments and the presence of chlorides explains the severity of the corrosion.The boxes were presumed to have been powder coated, yet microscopic and chemical analysis showed the coatings were probably applied as liquids. This was further supported by the thinning of the coatings at the sharp edges and corners of the boxes.     

Effect of Fe-Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe-Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe-Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn₁₃, FeZn₇ and Fe₃Zn₁₀ intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet-dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn²⁺ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.     

Effect of porosity of phosphate coating on corrosion resistance of galvanized and phosphated steels Part II: Evaluation of corrosion resistance

The effect of porosity of phosphate coatings on the corrosion resistance of ungalvanized (UG), electrogalvanized, and hot dip galvanized steels is evaluated in this study. The corrosion resistance of phosphatized and painted steel is related to the integrity and continuity of phosphate and paint layers, and pores in the phosphate layer affect the corrosion resistance of material. The porosity of the phosphate coating was evaluated by using the cathodic polarization electrochemical test. To evaluate the corrosion resistance of the phosphatized and painted steels, they were submitted to accelerated corrosion tests. As was expected, the creepback from the scribe increased with the increase in porosity. This behavior was evident for UG steel, but less evident for galvanized steels due to cathodic protection and/or barrier effect of the zinc coating.     

Corrosion resistance of steel with hot‐dipped galvanized zinc and zinc alloy coatings in seawater

Hot‐dipped galvanized zinc and zinc alloy coatings were used as protective metallic coatings for steel structures in seawater in China. Corrosion of the two coatings immersed in sea water in Qingdao and Xiamen for 6 years were introduced and analyzed, which provides a basis for further development and applications of these coatings in China. Tests proved that the anti‐corrosion performance of the hot‐dipped low alloy zinc coatings (aluminum content less than or equal to 10 wt%) is equal to or even lower than that of the pure zinc sheet, while the performance of the hot‐dipped high alloy zinc coatings is higher than that of the pure zinc sheet. The hot‐dipped high alloy zinc coatings can be further developed for optimal performance in the future.     

热镀锌层在模拟湿热酸性大气环境中的耐蚀性研究

目的研究Q420钢表面热镀锌工艺中,Zn和Zn-Al-Ni-RE合金镀层在酸性铜离子加速盐雾试验条件下的耐蚀性能。方法 Q420钢表面预处理后进行热镀锌,根据GB 6460—1986进行铜加速醋酸盐雾腐蚀试验,对比纯Zn镀层与Zn-Al-Ni-RE合金镀层的耐蚀性。结果 Ni,RE等元素的加入使镀层表面光亮,组织更加细密。在酸性铜离子加速实验进行到192 h时,纯锌镀层的腐蚀质量损失是合金镀层的2.7倍;72 h后纯锌镀层出现红锈,120 h后合金镀层出现红锈,说明Zn-Al-Ni-RE合金镀层比纯Zn镀层更耐腐蚀。结论通过适量添加Al,Ni与稀土元素,能使Q420钢合金镀层的耐蚀性能大幅度提高。     

Effect of Immersion Time and Cooling Mode on the Electrochemical Behavior of Hot-Dip Galvanized Steel in Sulfuric Acid Medium

In this work, we investigated the influence of galvanizing immersion time and cooling modes environments on the electrochemical corrosion behavior of hot-dip galvanized steel, in 1 M sulfuric acid electrolyte at room temperature using potentiodynamic polarization technique. In addition, the evolution of thickness, structure and microstructure of zinc coatings for different immersion times and two cooling modes (air and water) is characterized, respectively, by using of Elcometer scan probe, x-ray diffraction and metallography analysis. The analysis of the behavior of steel and galvanized steel, vis-a-vis corrosion, by means of corrosion characteristic parameters as anodic (β _a) and cathodic (β _c) Tafel slopes, corrosion potential (E _corr), corrosion current density (i _corr), corrosion rate (CR) and polarization resistance (R _p), reveals that the galvanized steel has anticorrosion properties much better than that of steel. More the immersion time increases, more the zinc coatings thickness increases, and more these properties become better. The comparison between the two cooling modes shows that the coatings of zinc produced by hot-dip galvanization and air-cooled provides a much better protection to steel against corrosion than those cooled by quenching in water which exhibit a brittle corrosive behavior due to the presence of cracks.     

Effect of molybdate post-sealing on the corrosion resistance of zinc phosphate coatings on hot-dip galvanized steel

The technique of post-sealing the phosphated hot-dip galvanized (HDG) steel with molybdate solution was addressed. The composition and corrosion resistance of the improved phosphate coatings were investigated by SEM, EDS, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements, and neutral salt spray (NSS) test. The results showed that molybdate films were formed in the pores of phosphate coatings, and the compact and complete composite coatings composed of phosphate coatings and molybdate films were formed on the zinc surface, resulting in that both the anodic and cathodic processes of zinc corrosion were inhibited remarkably; the corrosion protection efficiency values were increased; and the electrochemical impedance values were enhanced at least one order of magnitude. The low frequency impedance values for the composite coatings were increased at the initial stages of immersion in 5% sodium chloride solution, indicating the self-repairing activity of the composite coatings.     

Wasserstoffinduzierte Spannungsrißkorrosion an unverzinkten und verzinkten Baustählen

Hydrogen induced stress corrosion cracking of non galvanized and galvanized construction steels The processes of atmospheric corrosion and corrosion in collected water which may lead to hydrogen induced stress corrosion cracking of high-strength reinforcing steels in casing tubes before injection with concrete are discussed. Hydrogen uptake during corrosion occurs in weakly acid solutions as well as in neutral or alkaline aqueous solutions. The hydrogen uptake by proton discharge in acid solutions decreases with increasing pH of the electrolyte. Hydrogen can also be absorbed in neutral to weakly alkaline solutions if steels are plastically deformed and water reacts with the fresh iron surface. In alkaline solutions, hydrogen uptake is possible if, at the generally passivated steel surface, localized corrosion (pitting or crevice corrosion), local galvanic cells and a sufficient decrease in the pH of the pit electrolyte occurs. In the case of galvanized steels with damaged zinc layers, hydrogen uptake may result from the cathodic polarization of the free steel surface by zinc dissolution. The absorbed hydrogen interacts with the microstructure of the steels and weakens the bonds between the iron atoms. The influence of the microstructure of high-strength steels on the fracture behaviour is discussed on the basic of the so-called decohesion theory.     

镀锌钢构架在北美住宅环境中的耐蚀性能

钢结构住宅是未来住宅建筑的发展方向之一,而住宅钢结构防腐蚀寿命也成为钢结构住宅推广的关键因素之一。国际铅锌研究组织项目"住宅镀锌钢构架在实际住宅环境下的耐蚀性研究"通过选取北美4种典型的住宅环境,对锌及锌合金镀层钢铁试样进行了连续7年的暴露试验,并对镀锌钢构架在不同住宅环境中的腐蚀速率进行了评估。公布的研究结果表明,镀层质量为200~270 g/m2的镀锌钢以及质量相当的锌合金镀层钢试样在4种环境下腐蚀速率均非常小,以所有试样中的最大腐蚀速率来评估镀锌及锌合金钢试样在4种环境下的使用寿命可达258~964年,平均574年。在住宅设计使用年限内,采用镀锌钢结构不会因腐蚀而使得钢结构性能降低。镀锌钢可以作为一种可持续性的建筑材料使用。     

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.‐%.     

明度差法研究电镀锌铬酸盐钝化膜的耐蚀性

用明度差法研究了电镀锌未钝化及涂覆型钝化试片在湿热实验中的初期腐蚀行为。通过考察明度差△Ｌ随时间及温度的变化关系，发现未钝化镀锌层的初期腐蚀规律符合对数方程，而钝化后的符合Ｗａｇｎｅｒ方程，即印化膜的存在改变了腐蚀的历程，使钝化膜内锌离子的外护散或膜外氧的内扩散成为腐蚀的控制步骤，有效地提高了镀锌层的耐蚀性，采用明度差法可以反映电镀锌钢板腐蚀的程度。     

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 feuerverzinkter Baustähle in überwiegend sandhaltigen Böden

Corrosion behaviour of galvanized steel in mainly sandy grounds In mainly sandy grounds with different portions of fine parts < 0,06 mm (2,5–20,4%) specimens of ungalvanized and galvanized steel were stored outside and in the laboratory. Besides the composition of the ground, the salinity and the temperature of the ground were varied too. The corrosion rate and, for ungalvanized specimens, the behaviour to pitting corrosion were determined. Furthermore the factors characterising the corrosion behaviour such as specific resistance of soil and corrosion potential were investigated continuously. The loss in weight of metal was much greater for ungalvanized than for galvanized specimens and increased for ungalvanized specimens with an increasing portion of fine parts in the ground. Additions of salt at the beginning of the tests produced an increased amount of metal wastage, but for galvanized specimens they only had an influence upon initial corrosion. The increased removals of material started since contents of 3 · 10^?3 MolCl^? + SO/kg. If salts were added to the ground after 2 years (after the formation of a surface layer), they increased the wastage of material for ungalvanized but not for galvanized specimens. Apart from ungalvanized bars in the soil with a fineness portion of 20,4%, corrosion, after an acceleration at the beginning, slowed down owing to the formation of a surface layer. Ungalvanized specimens were attacked by a strong pitting corrosion and that more in aerated than in dense and, thus, water-containing grounds. The additions of salt accelerate more an uniform corrosion of material than a pitting corrosion. As for galvanized specimens after a local removal of zinc under extreme conditions the steel base had been hardly corroded away. The parts free of zinc were protected cathodically by the still existing zinc. The corrosion of steel depends upon the temperature: by increasing the temperature from 4 to 20°C increases of corrosion up to 100% were stated. As for galvanized surfaces temperature has only a small influence upon corrosion.     

Microstructure of the coating and mechanical properties of galvanized chromium-rich martensitic steel

Protecting the modern high-strength steels against corrosion is a challenge because the coating technology must be compatible with forming and must preserve the mechanical performances. Batch galvanizing after hot stamping could provide a simple solution to this complex problem. A commercial high-strength martensitic steel containing 13 wt.% Cr, 0.35 wt.% Si, 0.3 wt.% Mn and 0.15 wt.% carbon has been galvanized with a commercial zinc alloy. Galvanizing produces a ~ 15 μm thick coating that is bright, continuous and metallurgically bonded. The intermetallic layer is made of ? crystals, which forms an open 3-dimensional structure. Tin, nickel and aluminium are found able to moderate the Sandelin effect. Comparison with other steels galvanized the same way indicates that chromium slows down the kinetics of the metallurgical reaction. Chromium distributes both in the ? and η phases, and follows a diffusion-like profile in the coating. The nickel from the alloy concentrates in the Fe-Zn intermetallic compound. Aluminium segregates at the surface and interface. It also provides a gettering effect that fixes silicon in sub-micron particles dispersed in the ? and η phases. Tensile experiments and fatigue tests demonstrate that the mechanical performances of the martensitic steel are preserved after coating. Comparison with similar experiments performed on a TRIP800 steel indicates that using galvanized martensitic steel is best worth in static applications.     

Die Korrosionseigenschaften von feuerverzinktem Stahlblech in warmen weichen Wässern mit CO2- und/oder O2-Spülung

Corrosion properties of hot dipped galvanized steel plating in soft warm water with CO₂ and/or O₂ flows Research has been carried out on hot dipped galvanized plates in warm (70° C) diluted NaCl and Na₂SO₄ solutions with a conductivity of 300 μ S/cm at room temperature as a function of the composition of the flowing gas. When exposed to oxygen, the specimen plates show marked pit corrosion already after a few days. An attack on the zinc layer is followed by a local attack on the bared surface of the steel. Cathodic protection by sacrificial magnesium anodes, whilst reducing the corrosion rate, gives rise to needle-prick pitting corrosion on the zinc layer. The only means of stopping corrosion completely is the application of external current protection with several amps pr. sq. metre. With carbonic acid flow, the zinc layer is removed homogeneously, the corrosion rate being of the order of 0.07 μ/h. In the case of gassing with a 1 : 1 mixture of carbonic acid and oxygen, a corrosion-inhibiting covering layer of hydrozincite is formed. Hot dipped galvanized specimens with a zinc cover of approx. 47 μ showed no signs of corrosion after 4350 hours. In no case was it possible to observe a reversal of the potential of the iron-zinc element. The pitting corrosion encountered with oxygen flow must be ascribed to the effect of Evans' elements.