Engineering Failure Analysis 33 (2013) 381–386

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Engineering Failure Analysis

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Corrosion failure analysis of galvanized steel pipes in a waterirrigation system

1350-6307/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.engfailanal.2013.06.024

⇑ Corresponding author. Tel.: +55 16 33518507; fax: +55 16 33518258.E-mail addresses: carlosdrovere@hotmail.com, carlosdrovere@gmail.com (C.A. Della Rovere).

C.A. Della Rovere a,⇑, R. Silva a, C. Moretti b, S.E. Kuri a

a LC/DEMa/UFSCar, Corrosion Laboratory, Department of Materials Engineering, Federal University of São Carlos, Rodovia Washington Luis, Km 235,13565-905 São Carlos, SP, Brazilb CCDM/DEMa/UFSCar, Laboratory of Electron Microscopy, Center for Characterization and Development of Materials, Department of Materials Engineering,Federal University of São Carlos, Rodovia Washington Luis, Km 235, 13565-905 São Carlos, SP, Brazil

a r t i c l e i n f o

Article history:Received 5 June 2013Received in revised form 22 June 2013Accepted 25 June 2013Available online 3 July 2013

Keywords:Galvanized steel pipesIrrigation systemCreek waterAcid corrosionImpingement

a b s t r a c t

The premature failure of a water irrigation system made of galvanized steel pipes installedin the orange orchards of a citrus farm located in the interior of the state of São Paulo, Brazil,was investigated by onsite visual inspection, microstructural characterization of the Zncoatings based on SEM/EDS analysis, and chemical and physical analysis of the water usedin the system. The irrigation water comes from a creek that runs through the farm, and thefirst pipe leakages were detected after the system had been in operation for only 2 years.The findings of this investigation indicate that the failure of the pipes before their expectedservice lifetime was caused by acid corrosion, which rapidly attacked the Zn coating of theinner walls of the pipes, exposing the entire steel surface in the first months of use. Thedeterioration of the pipes was intensified by the mechanical wear resulting from a substan-tial amount of suspended solids in the flowing water. In addition, the SEM/EDS analysis ofthe Zn coating suggested that some points of the galvanized pipes exhibited manufacturingdefects, further impairing the corrosion performance of these pipes.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Only in two specific periods in history, during the sugar and coffee cycles, has Brazil been in strong control of the globaltrade of an agricultural product, as is currently the case in the global citrus market. Today, 70% of the orange juice consumedin the world is grown or processed in this country. This sector employs approximately 400 thousand people, generating an-nual export revenues of 1.5–2.5 billion American dollars and accounting for more than half of the orange juice produced inthe world and 85% of the juice concentrate traded on the international market [1].

In addition to Brazil, the United States, Mexico, Italy and Spain rank among the world’s major citrus producers. One of themain factors that have enabled Brazil to achieve its high citrus production volume is its tropical and subtropical climate. Theideal temperature range for orange orchards is 22–33 �C, and an adequate annual rainfall is around 2100 mm well distrib-uted throughout the year. However, since citrus production is strongly affected by low rainfall or its irregular distribution,eventual water deficits during periods of drought must be corrected through artificial irrigation [2].

To offset the water deficit in their orange orchards, most Brazilian farmers use a drip irrigation system usually made ofzinc-coated (galvanized) steel pipes.

Zinc (Zn) coatings are generally regarded as the most economical means of protecting steel against corrosion. Theyimprove the water corrosion resistance of steel by two methods, barrier protection and galvanic protection. In barrier

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protection, the Zn coating, which separates the steel from the corrosive environment, will corrode first before the corrosiveenvironment reaches the steel. In galvanic protection, zinc is less noble or anodic to iron in ambient conditions, and willcorrode sacrificially to protect the steel substrate, even if some part of the steel is exposed, such as cut edges or scratchesin the coating. In addition, Zn coatings are normally very corrosion resistant in natural waters due to their ability to formdense, adherent basic corrosion product films on the metal surface. For this reason, galvanized steels are widely used innatural waters and have a fairly long lifetime. However, it should be kept in mind that when they are subjected to moreaggressive environments their service life can be substantially decreased [3–6].

According to the American Galvanizers Association [7], the application of galvanized steel must be carefully designed be-cause the various types of water found in nature differ greatly. Moreover, corrosion rates are very difficult to predict becauseseveral factors affect the corrosion of metals in aqueous environments, such as pH levels, oxygen content, temperature andfluid velocity, to name but a few. Despite these difficulties, however, galvanized steel is one of the best methods to protectagainst corrosion caused by natural waters and can perform flawlessly for 8–12 years. Nevertheless, the first step in any deci-sion to use galvanized steels for a given application is to identify the characteristics of the water to which they will beexposed.

This paper analyzes the corrosion failure of galvanized pipes used in a drip irrigation system installed on a Brazilian citrusfarm. Because the underlying steel structures were protected against corrosion by galvanized zinc coating, the contractorestimated the service life expectancy of the irrigation system at 10 years; however, countless leaks were detected alongthe water pipes after only 2 years of use. As a result, the farmer, who considered the job was poorly done and the inbuiltmaterial was not suitable, filed a lawsuit against the contractor. However, as mentioned earlier, the use of galvanized pipesin natural water is supported by the literature, so the contractor decided to conduct a thorough investigation to discover thecause of the leaks. Therefore, the purpose of this paper is to elucidate the reasons for the decreased service life of these gal-vanized steel water pipes.

2. Sampling and field findings

The drip irrigation system, which was made of galvanized steel water pipes, was installed in the orange orchards of a cit-rus farm located in the interior of the state of São Paulo, Brazil, approximately 2 years before the first leaks were detected.The system was fed from a creek that runs through the farm and distributed water to all the orchards on this farm. Fig. 1a andb, respectively, depict a portion of the creek from which water was collected and the water distribution system.

A visit to the area of the farm where the failure occurred was required for a visual inspection. Fig. 2 shows three photo-graphs of portions of the irrigation pipes most severely damaged by corrosion. As can be seen, the inner surface of the Zncoating of the pipes was strongly attacked and its rust protection completely destroyed, while the Zn coating on the outersurface showed no significant attack. Careful removal of sediments and corrosion products revealed fairly uniform corrosionof the entire inner surface of the pipes, as well as some pits of various depths in a few areas, indicating that the corrosionprocess which caused the leakage occurred from the inside out. In addition, some failures of the system’s components,mainly at pipe elbows, (Fig. 2c) appeared to be directly related to impingement [8–10]. Fig. 2c clearly shows that the pipeends were relatively unaffected, but the metal failed where the direction of the water flow was diverted.

Since the creek water was not treated prior to its distribution, the next step in this investigation was to analyze the char-acteristics of the water (particularly in terms of its pH, hardness and suspended solids content). Water samples were col-lected at five different points of irrigation system and were analyzed by two different laboratories. All the samples werecollected in bottles, which were previously washed and cleaned in the laboratory. At the sampling sites, the bottles were firstwashed with the same water to be analyzed (thus removing any traces of detergents or residue) and then filled to the top andtightly sealed. In addition, samples of unused galvanized pipes were extracted for microstructural and compositional anal-ysis by scanning electron microscopy–energy dispersive spectroscopy (SEM/EDS).

Fig. 1. (a) The creek that fed the irrigation system, and (b) the water collector and distribution system.

Fig. 2. Main field findings: (a) heavily corroded inner surfaces of pipes; (b) leakage occurring from the inside out; (c) impingement.

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3. Results and discussion

3.1. Analysis of the galvanized Zn coating

An unused pipe was analyzed by SEM to verify the quality of the Zn coating on the galvanized pipes. The SEM micrographsin Fig. 3 depict a cross section of the unused galvanized pipe, showing details of the inner and outer Zn coatings and theirmicrostructures. Note that both the inner and outer Zn coatings are quite uneven, presenting an average thickness of around89.6 lm (measured by SEM at 5 different points). It is well known that Zn coatings can be applied by several methods, e.g.,hot-dip galvanization, electrogalvanation, metallization, mechanical plating, painting, etc. These methods produce coatingswith different types of structures and characteristics, such as thickness, microstructure, uniformity, density, composition,surface finish and type of bond between Zn and steel substrate [11]. The characteristics of the Zn coatings shown in Fig. 3indicate that they were produced by hot-dip galvanization. In this process, many chemical reactions take place betweenthe Fe of the workpiece and the molten Zn, forming distinct layers of intermetallic compounds and producing a metallurgicalbond between the Zn coating and the steel substrate. Fig. 4 depicts the typical morphology of a Zn coating on steel producedby hot-dip galvanization, which generally consists of four well defined layers. The first three layers are composed of inter-metallic compounds [gamma (C), delta (d) and zeta (f)], which vary in Zn content (with increasingly higher Zn content ineach consecutive layer on the steel), with the outermost layer [eta (g)] typically composed of pure Zn [3,12–14]. However,note the clear and sharp contrast between this figure and Fig. 3c, in which the microstructure contains numerous regionsdarkened by the presence of sulfur-rich compounds, as indicated by the EDS microanalysis performed in a more detaileddark region (Fig. 3d). In general, the incorporation of sulfur in Zn coatings applied by hot-dip galvanization on steel is linkeddirectly to residual oil on the workpiece surface, resulting from incorrect or inadequate surface preparation prior to galva-nizing. An analysis of Fig. 3c also suggests that, at some points, the adhesive bond between the Zn coating and the steel sub-strate was impaired by the presence of sharp cracks at the steel substrate/zinc coating interface.

Although the corrosion performance of the Zn protective coating was undoubtedly affected by these imperfections atsome points of the galvanized pipes, it should be kept in mind that Zn also acts as a sacrificial anode [3–6]. This means that

Fig. 3. Analysis of the galvanized Zn coating: (a) unused galvanized pipe analyzed by SEM/EDS, (b) cross section view, (c) Zn coating microstructure andcharacteristics, and (d) EDS microanalysis.

Fig. 4. Typical microstructure of a hot-dip galvanized Zn coating [4].

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even if some parts of the Zn coating became detached prematurely, the exposed steel would still be protected cathodicallyfrom corrosion by the remaining Zn coating. This fact does not allow the premature failure of these pipes to be attributedmainly to the aforementioned manufacturing defects.

3.2. Chemical and physical analysis of water

It is well known that natural waters are seldom pure, and that their characteristics are strongly influenced by the topog-raphy and geological composition of the areas through which they run. Thus, as water flows across the land’s surface, it dis-solves some of the minerals (and gases) from the soil, and the force of the flowing water carries along finely divided particlesof eroded soil and organic matter in suspension [15,16]. Therefore, since the creek water was not pretreated prior to its dis-tribution through the irrigation system, it was subjected to a chemical and physical analysis to measure its pH, hardness andsuspended solids content. Table 1 describes the results of the water analysis made by the two laboratories, which deter-mined that the water circulating in the irrigation system is slightly acidic, with an average pH of 5.33 and hardness of around18.4 ppm (as CaCO3), indicating that it is soft water [17,18].

According to the literature [4–6,16], in the absence of reducing or passivating agents, the corrosion of Zn in aqueous solu-tions is determined primarily by their pH. Fig. 5 shows the variation of the Zn corrosion rate, measured in centimeters ofpenetration per year (cm/year), as a function of pH. Note that the corrosion penetration rate of Zn is relatively low in thepH range of 7–12, which is mainly due to the formation of a protective film of corrosion products on the Zn surface; however,the penetration rate increases considerably on either side of the 6–12 pH range because Zn is an amphoteric metal that

Table 1Results of the analysis by two different laboratories of the water collected from five distinct points of the irrigation system.

Position Laboratory pH Hardnessa (ppm) TSSb (mg/L)

1 1 5.15 18 202 1 5.62 21 213 2 5.4 19 194 2 5.2 16 225 2 5.3 18 19Average – 5.33 18.4 20.2

a Water hardness in ppm of calcium carbonate (CaCO3).b Total suspended solids in the water.

Fig. 5. Zn corrosion rate in water as a function of pH [5].

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dissolves readily in acidic or strongly alkaline solutions. This indicates that galvanized pipes are suitable only in water withnear-neutral pH; in other situations, the pH level of circulating water would require strict control because variations couldoccur, leading to high Zn corrosion rates. In this graph (Fig. 5), also note that, at a pH level of about 5.3, which is the averagepH of the water circulating in this irrigation system, the Zn coating underwent considerably uniform corrosion, with a pen-etration rate of 0.05 cm/year (20 mpy). Based on this penetration rate and the highest and lowest Zn coating thicknessesmeasured by SEM, it is possible to estimate the durability of the Zn coating on the galvanized pipes of this irrigation system.Table 2 presents the estimated durability of the Zn coating. Note that the Zn coating shows low durability, and that the steelsurface at some points of the pipes would already be exposed to the circulating water after just 1.5 months of use.

In addition, since the water under analysis is soft (18.4 ppm as CaCO3), the corrosion attack described above is furtheraggravated because water hardness is another important variable in Zn corrosion. The literature indicates that the corrosionrate of Zn in hard water may be as low as 15 lm/year (0.6 mpy), but corrosion attack is much more aggressive in soft water,and can reach 150 lm/year (6 mpy). Hard waters tend to be less corrosive toward Zn because they deposit a protective scaleon the metallic surface, while softer waters do not deposit these scales [5,6,16].

Table 1 also indicates that the creek water contains about 20 mg/L of total suspended solids, determined gravimetrically,indicating that considerable quantities of fine solid particles circulate through the irrigation system. As mentioned earlier,creek or river waters contain large amounts of suspended solids because they interact directly with the ground over whichthey flow, carrying along different particles such as eroded solids (sand), decaying vegetation, solid pollutants and colloidaland suspended matter. Therefore, in pipes that carry flowing water, these suspended solids are flung against the inner wallsof the pipes, damaging or mechanically removing the corrosion product films. This erosion corrosion process allows moremetal to go into solution, increasing the wear of the pipes’ inner walls. Therefore, when water flowing through pipes containssuspended solid particles, they must be removed by filtration to reduce this type of deterioration.

Table 2Estimated durability of Zn coating as a function of coating thickness measured by SEM,considering a corrosion penetration rate of 0.05 cm/year.

Measurement Coating thickness (lm) Durability (months)

Lowest thickness 62.89 1.51Highest thickness 116.37 2.79Average thickness 89.63 2.15

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Moreover, since the total volume of water required by these orange orchards is about 5800 m3/day and the creek watercontains about 20 mg/L of suspended solids, the water flowing through the irrigation system contains approximately116 kg/day of suspended solids. The collision of this large volume of solid particles against the inner walls of the pipes causeshigh mechanical wear, diminishing the durability of the pipes. As shown earlier in Fig. 2c, some cracks, particularly at thepipe elbows, occurred due to impingement, which was probably greatly intensified by the substantial suspended solids con-tent in the flowing water. Therefore, the relatively low pH of the flowing water and the erosive effect caused by suspendedsolid particles are probably the main factors that caused the irrigation pipes to fail before they reached their expected servicelifetime.

4. Conclusions

The results of this investigation indicated that the use of galvanized pipes was not a suitable choice for the natural irri-gation system of this citrus farm. Galvanized pipes are suitable only when the pH of the natural water is about 7 or moder-ately basic (between 8 and 12). Therefore, such pipes should not have been used for this application, whose water has anaverage pH of 5.3. Protective zinc coating is not resistant to corrosion in acid pH and therefore its service life under suchconditions is very limited. In general, the durability of galvanized pipes in natural water can reach up to 12 years, but in thiscase, the irrigation system began to leak after only 2 years of use.

In addition to its acid pH, the natural water used in this irrigation system contains substantial amounts of suspended sol-ids, which contribute to aggravate the deterioration of galvanized pipes due to erosion corrosion and impingement attack.

The SEM/EDS analysis also indicated that the Zn coating exhibited some manufacturing defects, which further impairedits corrosion performance, although these defects were probably not the main cause of the system’s premature failure.

The recommended solution to solve the problem is to install a water treatment plant and adjust the pH level to the properrange via chemical treatment before the water is distributed. In addition, a filtration system is required to reduce the water’ssuspended solids content.

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