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Научни трудове на Съюза на учените в България - Пловдив. Серия В. Техника и технологии, т. XIV, ISSN 1311-9419 (Print), ISSN 2534-9384 (On- line), 2017. Scientific Works of the Union of Scientists in Bulgaria-Plovdiv, series C. Technics and Technologies, Vol. XIV., ISSN 1311-9419 (Print), ISSN 2534-9384 (On- line), 2017.
ИЗСЛЕДВАНЕ НА КОРОЗИЯ НА ПОКРИТИЯ ЦИНКОВИ ПО ВЪНШНИ УСЛОВИЯ В МИТРОВИЦА Ali H. Sadiku1, Mursel Rama1, University of Mitrovica, Faculty of Geosciences, Department of Materials
and Metallurgy, PIM Trep?a, 40000 Mitrovica, Kosovo Е-мейл: alisadiku@hotmail.com
STUDY OF CORROSION OF ZINC COATINGS IN EXTERNAL CONDITIONS IN MITROVICA Ali H. Sadiku1, Mursel Rama1, University of Mitrovica, Faculty of Geosciences, Department of Materials
and Metallurgy, PIM Trep?a, 40000 Mitrovica, Kosovo e-Mail: alisadiku@hotmail.com
ABSTRACT:
Zinc coating produced with hot- dip galvanization process has a number of advantages over other processes of corrosion protection. This conclusion comes from the fact that zinc coatings a long life, technological process from a technical aspect is quite simple as well ecologically acceptable for the environment.
That zinc coatings are environmentally very acceptable, easily can be verified by theoretical data as well as from practical results for these coatings.
In this paper are presented results of gravimetric measurements of corrosion rate of zinc coatings applied with hot-dip process in the outdoor and laboratory conditions. Also in these samples was determined the change of the electrode potential in respect of calomel saturated electrode (SCE).
Obtained results shows that zinc coatings are resistant to corrosion, corrosion products are compact and do not pollute the environment.
Key words: zinc, zinc coating, corrosion, environment pollution.
Introduction
The main use of zinc is based on its properties resistant to atmospheric corrosion. Over 7 million t/year of zinc produced worldwide. Approximately 50% of this amount is used as zinc coatings, to protect steel from corrosion.
Zinc coatings are used primarily to protect metal parts from atmospheric corrosion. Atmospheric corrosion is defined by Schilcor 1965, as the process of corrosion in air temperature of -20°C up to 70 °C, in outdoor or indoor conditions [5]. The atmosphere can usually be classified as industrial atmosphere, urban and rural, classification that corresponds to the concentration of
high, medium and low SO2 contents. The fourth type of atmosphere, it reflects the presence of chlorides coastal sea.
Overall zinc coatings in atmosphere corrode similarly as solid zinc. Corrosion of coatings in the air is almost in proportion with mass losses and time. Since the thickness of the protective film on zinc may increase over time, in rural and marine atmosphere in some cases, then the life expectancy of zinc can be increased in proportion to the thickness.
According to the pH-potential diagram (Pourbaix diagram) for Zn, in aqueous solutions Zn is unstable and tends to disolve with release of hydrogen throughout the all intervale of pH value. In solutions with pH between 8.5 and 12 zinc can be covered with hydroxide film, which has the effect of inhibiting the disolve of zinc in the absence of chemical species which can form solubile zinc complex. Zinc sacrificial coatings protect the iron in three ways: Isolation (barrier) coating layer, secondary isolation (barrier) of corrosion products and galvanic protection. In fig. 1 is schematically shown galvanic protetction of zinc coatings.
Nonuniform surface, generate the formation of corrosion cells. These irregularity in protection film resulting from defects in the surface oxide film, the local distribution of elements and differences in the structure of crystalline phases. Many corrosion cells formed on the surface of the coating accelarate corrosion when exposed to the atmosphere. During the time, corrosion products gradually become stable layer, and corrosion rate decreases and remains constant. This barrier of protection against corrosion constantly regenerated in long time.
Eiectroiite
Zi(OH;2
P^mmc
¿7Z1 ijcrdoatirig ^,J
>{*#'///Á
Fe
ioc- Fe -without zinc coating ioc- zinc coatings ■o ioc -Fe with zinc coating
Figure 1. Galvanic protection of zinc coatings [1], [6].
Materials protected by zinc coatings are used in industrial field quite as well as construction material. Widespread use of this material in Kosovo and Mitrovica outdoors based on several factors: good constructive properties and anti-corrosion, economic cost and its availability.
Mitrovica is now a post-industrial city. In the past, up to 90 years of the last century was a typical industrial atmosphere, very aggressive due to the high content of SO2. This high content of SO2 in the atmosphere was due to emissions from plants: zinc electrolysis, fertilizers and lead batteries.
But after 1999, when industrial activity in Mitrovica is, completely stopped, the atmosphere is no more of SO2 content, and consequently is no so aggressive. This atmosphfere can now be
considered as urban without aggressive content of SO2, since concentration of SO2 in atmosphere is in same level as are in non industrial area.
To get an overview of corrosion of zinc coatings in Mitrovica atmosphere, were made researched of corrosion of zinc coatings in a number of samples in the externale environment (atmosphere).
Materials and metodology
For the experiment were taken five samples of galvanized steel sheet, forms and sizes of these samples are given in fig. 2. The weight of the zinc coating has been 135g / m2 respectively 9,45 ^m thickness of the coating. The thickness of the coating is determined by the gravimetric method according to standard EN 10142.
Figure 2. Dimension of samples: a)gravimetric measurements, b) electrochemical measurements.
Ribs of galvanized sheet samples are painted with epoxy paint. In two samples surface of coatings is damaged on the one in the form of the letter X (i.e. by diagonal), while to another in an irregular manner (fig. 3).
Figure 3. Prepared samples before exposure.
Samples are cleaned with ethanol, dry and weight in the analytical balance. Prepared samples were placed in the outer atmosphere conditions, on real natural conditions. Exposure time was 69 days or 1656 h.
Forme and dimension of samples for measuring of electrode potential of the zinc coating are given in fig.2.
Potential measurement is done in natural water (rain) and water (rain) in which the pH value was 5.5 and 8.5. The pH value of the solution (water) is adjusted by adding H2SO4 and NaOH, and measured with pH meter. Electrode potential has determined against SCE electrode (Calomel Saturted Elctrode) and measured with digital potentiometer "Iskra MI 7030".
Results and discusion
Throughout the period of exposure of samples to external environment changes are recorded and photographed the surfaces of samples. To samples in which the zinc coating has been damaged, it presented iron corrosion products with the red-orange color and not compact. This surface is corroded with time increased (fig. 3). In the samples with zinc coatings without damaging the change observed in the clothing color of the zinc layer. Layer of zinc coating in beginning is shine and bright, and after a week starts to lose brilliance, and at the end of the exposure period almost the entire surface of the coating has the dull gry color (fig. 3).
Based on observed changes we can conclude that in surface of cotings is formed thin film of zinc corrosion product, and this tightly-adherent corrosion film is called a passivation layer. The formation of the passivation layer is essential to achieving long-life with a galvanized product. Zinc coatings resist atmospheric corrosion by forming protective film of basic salts, mainly carbonates. The most acceptable formula of this corrosion product is 2ZnCO3-3Zn(OH)2 [4]. The critical time to form the passivation film generally takes 4 to 12 weeks to passivate [3]. Zinc layer in the atmosphere is subject to corrosion, which can be described with the following reactions: Oxidation: 2Zn + O2 ^ 2ZnO
Hydratization: 2Zn + 2H2O + O2 ^ 2Zn(OH)2
Carbonation 5Zn(OH)2 + CO2 ^ 2ZnCO3-3Zn(OH)2 + 2H2O
Environmental conditions can inhibit the formation of the protective film, leading to the formation of soluble product of corrosion and can cause zinc attack quickly. Chloride and nitrogen oxides typically have little effect, but the effect increases in combination with sulphates Frequency and duration precipitation, drying time, the daily temperature fluctuations and seasonal and the composition of the atmosphere are the factors that affect the rate of corrosion.
Figure 4. Samples after exposure in external atmosphere. Calculation of corrosion rate
After the corrosion process, samples are first cleaned with water, ethanol and after drying was measured weight of samples (m2). Corrosion speed is calculated according to equation:
Am m1 - m2
rate = J^ = j _ t
Where: mp m2 - sample weight before and after test (g); Am - change in weigt of sample; S - the surface of the sample in cm2 (300 cm2) and t - the time of the experiment sample in solution (1656 h).
Calculating corrosion rates for each samples are given in follwing table. Table 1. Corrosion rate of zinc coating samples.
Testing Sample/ mi m2 Weight Time Corrosion rate
environmet [g] [g] change, [g] [h] [g/cm2/h]
Atmosphere 1 66 4402 67.306 0.8628 1656 1.7367 x10-6
Atmosphere 2 67 7131 67.3505 0,3626 1656 7.2987 x 10-7
Atmosphere 3 67 3565 67.4651 0.1086 1656 2.1859 x 10-7
Atmosphere 4 66 8187 66.3203 0.4984 1656 1.0032 x 10-6
Atmosphere 5 60 2964 60.0172 0.2792 1656 5.6199 x 10-7
Obtained results shows that the weight of galvanized sheet samples increased after the corrosion process (after the experiment), because corrosion products are compact and protect the metal from further corrosion. As noted in the general zinc protect steel by acting as a isolation barrier for the penetration of aggressive environmental subtanace to steel and galvanic protection. This protection is based on the fact that the corrosion of zinc products protect the base metal from corrosion, since the corrosion products are compact and do not leave the zinc coating.
Corrosion rate values of zinc coating in the external environment shows that the corrosion products are compact and protect it from corrosion (2.1859 x 10-7 g / cm2h). Also, the rate of corrosion of the samples that have been damaged (1.7367 x10-6 g / cm2h) is greater than the sheet intact (order 10-7 g /cm2h).
Is the most well to note that the nature of the corrosion products of zinc coatings does not pollute environment and are not dangerous to human health. This conclusion comes not only from these experimental results but this fact can be prove from the experience of using these coatings in common environmental conditions.
Standard electrode potential of zinc (-0.76 V agnaist SHE) is more negative in comparation with iron (-0.44 V), and this differnce is main factor which made zinc an ideal metal to protect steel from corrosion in external conditions. Corrosion potential of zinc coatings were measured during 420 h in different water solution. During measurment is shown that changes in value of the electrochemical potential during time are very small (fig. 5). This small difference of corrosion potential value in different environments shows that zinc coatings are an effective method of corrosion protection of steel structures.
Figure 5. Diagram potential- time, of zinc coating.
Conclusions
Based on data for zinc galvanic coatings with hot immersion method is very efficient in protection of metals from corrosion. Zinc metal is very suitable for the protection of metals (steel) from corrosion thanks to its electrochemical properties. About 60% of the annual production of zinc metal coating used to protect against corrosion.
Based on the results of corrosion rate we can conclude that zinc coatings zinc offer highly effective protection from corrosion in external environment.
Even under laboratory conditions where the corrosion process is accelerated, confirmed the efficiency of the zinc coatings by immersion in hot.
Zinc coatings and corrosion products are not hazardous to the environment. This results from the fact that the rate of corrosion coatings is too small, also their corrosion products are compact and insoluble in external environmental conditions.
Kosovo has the potential to develop the industry of corrosion protection by zinc coatings. This perspective is based on the fact that Kosovo has significant reserves of minerals of zinc. Also, in Kosovo there are significant industrial capacity for zinc coatings of immersion in hot ("Llamkos", "Vinex") which can be activated by finding serious investors.
References:
Prifti M. Gage, Z."Shkenca e Korrozionit" FSHN, Tirane, 2002.
Gage Z. "Mbrojtja nga Korrozioni", FSHN, Tirane, 2003.
"Zinc Coatings" Publication of American Galavnizers Association, 2007.
Porter, C. F. "Corrosion Resistance of Zinc and Zinc Alloys", CRC Press, New York, 1994.
Zhang, G. X. "Corrosion and Electrochemistry of Zinc", Spinger, New York, 1996.
Ali Sadiku , " Ndikimi i perberjes se banjes ne vetite e veshjeve te zinkut mbi gelik» Teme magjistrature, FSHN,Universiteti iTiranes, 2008.
"Hot-Dip Galavnizing for Corrosion protection of Steel Products", American Galavnizers Association, 2006.
