Evaluating Zn, Al and Al–Zn coatings on carbon steel in a special atmosphere

Evaluating Zn, Al and Al–Zn coatings on carbon steel in a special atmosphere

Construction and Building Materials 23 (2009) 1465–1471 Contents lists available at ScienceDirect Construction and Building Materials journal homepa...

524KB Sizes 0 Downloads 8 Views

Construction and Building Materials 23 (2009) 1465–1471

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Evaluating Zn, Al and Al–Zn coatings on carbon steel in a special atmosphere O. de Rincón a,*, A. Rincón a, M. Sánchez a, N. Romero a, O. Salas a, R. Delgado a, B. López a, J. Uruchurtu b, M. Marroco c, Zephir Panosian c a

Centro de Estudios de Corrosión. Facultad de Ingeniería-Universidad del Zulia Aptdo. 10482, Maracaibo, Venezuela Universidad de Cuernavaca, Mexico c CEPEL, Rio de Janeiro, Brazil b

a r t i c l e

i n f o

Article history: Received 16 January 2007 Received in revised form 30 June 2008 Accepted 1 July 2008 Available online 15 August 2008 Keywords: Zn Al and Al–Zn coatings Carbon steel Atmospheric corrosion

a b s t r a c t This paper presents a comparative evaluation of Al, Zn and Al–Zn coatings on carbon steel, exposed to a coastal-marine atmosphere. It is a very aggressive atmosphere with high wind velocities (corrosion–erosion rate  1.4 mm/year (55.12 mpy) for ASTM 1029 steel). Two flame spraying zinc coatings with pore sealers were also evaluated. ISO and ASTM standards were used for the evaluation. After a 2-year exposure the best performance was achieved by the flame spraying Zn/15Al alloy (85% Zn– 15%Al) with some damage of the coatings. But the one with a wash primer pore sealer did not show signs of damage. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Carbon steels have excellent mechanical properties. Although easy, low-cost manufacture makes their use attractive, their resistance to atmospheric corrosion is very low in most environments and progressive deterioration of their structure leads to rust formation and consequent loss of some of their mechanical properties. In environments with little atmospheric pollution, atmospheric corrosion or rust formation is insignificant. However, expansion in the chemical, oil, metallurgical and thermoelectric industries has increased atmospheric pollution notably, with generally greater aggressiveness on bare materials. So, because of their good performance in inclement weather, materials such as zinc, aluminum and alloys thereof are being used as protective coatings for carbon steels. The protective efficacy of non-alloyed steels by means of metallic coatings depends on its capacity: (1) to act as an isolating barrier between the surrounding atmosphere and the underlying metal, i.e. on its thickness, uniformity, adherence, lack of porosity and ductility and (2) to provide electrolytic protection for the steel, acting as a sacrificial anode should a cell or galvanic couple be formed. The latter situation could arise on cut edges or in areas where part of the metallic coating may accidentally be lost [1]. In theory, a metallic coating applied to a metallic substrate creates a continuous barrier that completely isolates the base metal from the environment. Unfortunately, it is very difficult to achieve * Corresponding author. Tel.: +58 261 755507; fax: +58 261 7598175. E-mail address: [email protected] (O. de Rincón). 0950-0618/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2008.07.002

it in practice, since discontinuities such as pores, pitting and fissures are produced from the very moment of application. Besides, as coatings are prone to damage during transportation, the galvanic action at the base of a pore or damaged area becomes an important factor when determining coating features [2]. Zinc, aluminum and alloys thereof are among the coatings most frequently used to prevent atmospheric corrosion. Steel, acting as the base metal, and zinc and/or aluminum and their alloys, acting as protective coatings, constitute an excellent combination that simultaneously provides, at low cost, the mechanical rigidity and resistance to corrosion required by a construction. The lifetime of zinc coatings generally depends on their thickness, being more or less independent of the application method (hot dipping, metallized coating or electroplating). Tests carried out in the United States have shown that a thin coating of 0.025 mm (0.984 mils) in suburban or rural areas lasts 11 years or more; 8 years in marine areas and scarcely 3 years in coastal areas [3]. Coating composition is of crucial importance for its lifetime. For example, a hot dipping zinc–iron alloy tends to corrode at a lower rate than pure zinc. The zinc acts as a sacrificial anode on the steel, affording it effective galvanic protection, thus preventing a direct corrosive attack on the steel. Weather resistance is an excellent feature of aluminum and many of its alloys. This is due to the formation of a continuous, tenacious and very adhesive, insoluble and strongly passivating film of hydrated aluminum oxide (Al2O3  3H2O) that constitutes an excellent barrier against atmospheric corrosion, even in acid media.

1466

O. de Rincón et al. / Construction and Building Materials 23 (2009) 1465–1471

Hurtado [4] proved that aluminum coatings protect the base metal by forming a continuous non-porous coating that isolates it from the corrosive medium, said coating being almost immune to attacks from this medium. Again, Munger [5] found that the protective action of aluminum involves the formation of compact and insoluble corrosion products that seal discontinuities or anodic sites (pores) in order to protect the base metal from the environment. However, in spite of its theoretical capacity for efficient galvanic protection of steel, the anodic behavior of aluminum is, in practice, totally annulled by the action of Al2O3 (a semi amorphous film with very little conductivity). So, although aluminum coatings are highly resistant to atmospheric corrosion, it is reduced to almost nil when it comes to galvanic corrosion [1]. Base coatings with Al–Zn alloys evidence outstanding resistance to atmospheric corrosion with galvanic protection. In rural and industrial atmospheres, their protection lasts considerably more than that of a pure-zinc coating, and they provide much more resistance to corrosion in marine environments than a pure-aluminum coating [1]. Essentially, the behavior of Al–Zn coatings is similar to that of aluminum coatings because they form an oxide film (characteristic of aluminum) that protects the material. This film, dendritic in structure, is formed in combination with the zinc in the alloy. A greater or lesser amount of Al or Zn in the alloy will define the behavior of the coating: either passive (greater amount of aluminum) or galvanic (greater amount of zinc). As with many other types of metallic coatings, thickness defines their lifetime. Worldwide, many efforts are being made to analyze the behavior of these coatings on a certain substrate under different exposure conditions. One of these is PATINA (Anticorrosive Protection of Materials in the Atmosphere) – the IberoAmerican Project belonging to Subprogram XV ‘‘Corrosion/Environmental Impact on Materials” of the CYTED (Ciencia y Tecnología para el Desarrollo [Science and Technology for Development]) Program. This project was approved by CYTED on the basis of the excellent results obtained from the MICAT (Mapa Iberoamericano de Corrosión Atmosférica [Iberoamerican Atmospheric Corrosion Map]) Program, in which 72 stations were installed in IberoAmerica to evaluate the behavior of bare metals (Fe, Al, Zn, Cu) [6]. This work presents some of the results obtained in La Voz Station, Venezuela, as participating member in the PATINA project. Its purpose is to establish the behavior of diverse types of metallic coatings on carbon steel exposed to the special atmosphere for 24 months’.

Table 1 Characteristics of zinc-based metallic coatings Coatings

Description

Non-continuous hot dipping galvanizing Flame spraying Zn/15Al

Three intermetallic phases, and one external layer of pure zinc

60

Zinc alloy with 15% Al. Highly rough external layer, with a laminate structure, on flattened metallic particle position. Surface treatment: wash primer Zinc with 15% aluminum. Highly rough layer with a laminate structure on a flat metallic particle position. Surface treatment: vinyl sealer Pure zinc coatings without intermetallic layers Pure zinc coatings with cromatized surface treatment Dendritic phases enriched in zinc (solid solution of aluminum and zinc)

70–130a

Flame spraying Zn/15Al

Electroplating Continuous hot dipping Continuous hot dipping Zn /5Al a

Thickness (lm)

150

5 20 25

Including pore sealer, non-uniform.

Table 2 Characteristics of aluminum-based metallic coatings Coatings

Description

Thickness (lm)

Continuous hot dipping Al/13Si

Aluminium alloy, with approximately 13% Si, conformed by a matrix of phases of silicon flakes Pure aluminum coating, with a highly rough layer with laminar structure Two layers: an external one conformed by a dendritic phase of aluminum and the internal one containing interdendritic phases with different compositions

40

Flame spraying Al Continuous hot dipping 55Al/Zn (Galvalume)

150 20

3. Sedimentable atmospheric dust content (ISO/DP 9225). 4. Environmental conditions (RH, T, wind velocity, precipitation, etc.)

2.3. Visual Inspection of probe surfaces

Nine metallic-coating systems were evaluated, adopting a structure of two groups of protective coatings (Tables 1 and 2) as a function of the different materials to be tested. A horizontal scribe was made in the coatings on the lower part of the upward facing side of the specimens. No international standard was found in the literature regarding the way to perform a scribe of this type in metallic coatings. In our case, the scribe of 1 mm in width and 6 cm in length was performed with the assistance of a tungsten carbide tipped scriber, reaching the steel base after several passes.

Visual inspection consisted of evaluating whether there was formation of corrosion products in the coating and the substrate, peeling, corrosion around the edges (form and type) and, if the probe has any scribe, whether there is any corrosion starting from it. Any atypical damage should be described in detail to enable subsequent identification/recognition thereof. Besides visual inspection of the metal coatings, scanning electron microscopy (SEM) was also carried out both inside and outside the scribe, as well as loss-of-coating-thickness rate (ISO 9226). A grid square was used to determine the percentage of corroded area in both the coating and the base steel. The grid square was placed on the probe once it was removed from the stack, and the number of squares covered by corrosion products on the coating and/or the base steel was counted. It is important to point out that 1 cm from the borders must be discarded in order not to consider corrosion by erosion.

2.2. Evaluation of meteorochemical agents

3. Discussion of results

2. Experimental methodology 2.1. Test probes

The meteorochemical agents evaluated were:

3.1. Meteorochemical evaluation of the atmosphere at La Voz Station

1. Chloride-ion content (ISO/DP 9225). 2. Sulfur compound content (ISO/DP 9225).

The atmosphere is classified as SPECIAL because of its high aggressiveness and strong wind incidence (Table 3), with corrosion

1467

O. de Rincón et al. / Construction and Building Materials 23 (2009) 1465–1471

rates reaching 921.70 lm/year (36.29 mpy) for steel, 8.88 lm/year (0.35 mpy) for copper and 26.53 lm/year (1.05 mpy) for zinc [7], besides having a very high chloride content, which goes well beyond the range established for a marine atmosphere.

3.2. Visual inspection Tables 4 and 5 show the results of the visual inspection carried out on the zinc- and aluminum-based coatings exposed at La Voz Station, after 2-year exposure. The worst performance of these coatings was electroplating (zinc). During the first 3 months’ exposure, corrosion of the base steel surface was 100%, with total loss of the coating. This showed that this type of coating is unsuitable for aggressive environments such as the one at La Voz Station, due to

low coating thickness (5 lm/0.20 mils), typical of coatings lay down by electroplating (see Table 6). As can be seen in Table 4, a comparison of non-continuous and continuous hot dipping galvanizing shows that the behavior of the former is better both from the standpoint of general corrosion in the coating and erosion on the edge exposed to the prevailing winds (speeds of up to 7 m/s/22.96 ft/s with gusts of 9 m/s/ 29.53 ft/s). After 24 months’ exposure, the base steel is 100% corroded in the continuous galvanized coating with only 7% for the non-continuous (Fig. 1). This happens because the continuous galvanized coating does not have the Zn/Fe alloy layers of the noncontinuous galvanized coating (hardness 110 HK compared with 39 HK for continuous galvanized), since these layers are more resistant to the aggressive environment produced by the particleladen wind (corrosion–erosion).

Table 3 Average annual values of meteorochemical agents recorded at La Voz Station [7] Temperature (°C)

RH (%)

TDH (s) (h/month)

Rainfall (mm/year)

SO2 (mg/m2 d)

Cl (mg/m2 d)

Winds (m/s)

ISO 9225

34.70

92

528.5

398

29.85

374.76

7.20

>C5

Table 4 Visual inspection of the zinc base coatings during the exposition at La Voz Station Zinc-based coatings

Exposure time 6 months

12 months

18 months

24 months

Non-continuous hot dipping galvanizing (60 lm/ 2.76 mils)

 Corrosion–erosion of base steel and the coating in the left and lower edges by wind action  Formation of white colored corrosion products on the coating in the form of scales in 60% of the surface  Corrosion products formation on the coating at scribe visible at 10

 Corrosion–erosion of base steel and the coating in the left and lower edges by wind action  Formation of white colored corrosion products on the coating in the form of scales in 100% of the surface  Corrosion products formation on the coating at the scribe

 Corrosion–erosion of base steel and the coating in the left and lower edges, with peeling of the coating at the left edge (Lmax = 1.25cm/0.49 in.)  Formation of white colored corrosion products on the coating in the form of scales in 100% of the surface, and in 7% of the base steel  Formation of corrosion products on the coating and in the base steel at 100% of the scribe

Continuous hot dipping galvanizing (20 lm/ 0.79 mils)

 Formation of white colored corrosion products on the coating in the form of scales in 60% of the surface  Corrosion of the base steel and coating at the scribe

Flame spraying Zn/15Al and pore sealer (vinyl) (150 lm (5.91 mils))

 No damage at the plate

 Formation of white colored corrosion products on the coating in the form of scales in 100% of the surface  Corrosion in edges mainly in the left edge because of the direct action of the winds  Peeling of the coating in the left edge  Corrosion of the base steel and coating at the scribe  Corrosion of the base steel and the coating at the scribe visible at 10  Corrosion–erosion of the coating in edges because of direct action of the winds

 Corrosion–erosion of base steel and the coating in the left edge with the lost of coating because of direct action of the winds  Corrosion of the base steel of lower edges because of direct action of the winds  Formation of white colored corrosion products on the coating in the form of scales in 100% of the surface, and In 2% of the base steel.  Formation of corrosion products on the coating and in the base steel at scribe  Severe attack of the base steel and coating at the scribe  Formation of corrosion products of the coating in 100% of the surface  Peeling of the coating in the left edge (Lmax = 2.5 cm/0.98 in.)  Corrosion of the base steel in the left edge because of direct action of the winds (Lmax = 1 cm/0.39 in.)

Continuous hot dipping Zn /5Al (25 lm/ 0.98 mils)

 Corrosion of the left and lower edges of the coating and base steel because of direct action of the winds  Formation of white colored corrosion products on the coating in the form of scales in 50% of the surface  Coating corrosion at the scribe

 Corrosion–erosion of base steel and the coating in the left and lower edges because of direct action of the winds  Formation of white colored corrosion products on the coating in the form of scales in 70% of the surface  Corrosion of the base steel and coating at the scribe in 90%

 Corrosion of the coating at 25% of the surface  Localized corrosion of the base steel in edges and scribe visible at 10  Localized corrosion of the coating in edges and scribe  Corrosion of the base steel and coating at the left edge (Lmax = 2 cm/0.79 in.)  Formation of white colored corrosion products on the coating in the form of scales in 100% of the surface  Corrosion of the base steel and coating at the scribe

 Coating and base steel corrosion in 100% of the surface  Peeling of the coating in the left edge (Lmax = 3.5 cm/1.38 in.)  Severe corrosion of the coating and base steel at the scribe (100%)

 Pitting at 30% of the surface  Localized corrosion of the coating in edges and scribe  Localized corrosion of the base steel in edges and scribe visible at 10

 Steel corrosion (22%) and coating (100%) at the surface  Peeling of the coating at the left edge because of direct action of the winds  Corrosion–erosion of the steel and coating at the edges because of the direct action of the winds reaching 2 cm (0.79 in.)  Corrosion of the base steel at the scribe reaching 0.8 cm (0.32 in.)

Lmax: the maximum length affected from the edge (of the coating and/or base steel) exposed to the direct action of the preferential winds.

1468

O. de Rincón et al. / Construction and Building Materials 23 (2009) 1465–1471

Table 5 Visual inspection of the aluminum-based coatings during the exposition at La Voz Station Aluminum-based coatings

Exposure time 6 months

12 months

18 months

24 months

Continuous hot dipping Al/13Si (40 lm/1.57 mils)

 Generalized corrosion of the coating at the scribe  Localized corrosion of the coating at the edges

 Generalized corrosion of the coating at the scribe  Localized corrosion of the coating at the edges

Flame spraying Al (150 lm/5.91 mils)

 Low corrosion of the base steel and coating at the scribe  Localized corrosion of the coating at the lower edge (very low)  Corrosion of the coating at 70% of the surface visible at 10  Corrosion–erosion of the base steel and left edge and scribe of the coating

 Corrosion of the base steel at the scribe  Localized corrosion of the coating at the edges

 Base steel corrosion at the scribe  Generalized corrosion of the coating at the scribe with a maximum reach of 2 cm (0.79 in.)  Corrosion–erosion of the coating at the edges with a reach maximum of 1 cm (0.39 in.)  Severe corrosion of the coating in 100% (Lmax = 1 cm/0.39 in.) and base steel in 1% of the scribe  Corrosion of the coating at the coating in edges and scribe

 Pitting at 100% of the surface  Corrosion of the base steel in localized areas of the scribe and edges  Severe corrosion of the coating at 100% of the scribe (Lmax = 2.2 cm/ 0.087 in.)  Pitting at 100% of the surface  Corrosion of the base steel and coating in edges and scribe  Localized corrosion of the coating at edges

 Corrosion–erosion of the base steel (Lmax = 1 cm/0.39 in.) and coating (Lmax = 2.4 cm/0.94 in.) in edges  100% corrosion of the coating at the scribe (Lmax = 1.5 cm/0.59 in.)

 Corrosion–erosion of the base steel (Lmax = 1.5 cm/0.59 in.) and coating (Lmax = 3 cm/1.18 in.))in edges  Corrosion in 100% of the scribe (Lmax = 2 cm/0.79 in.)  Pitting at 100% of the surface

Continuous hot dipping 55Al/Zn (Galvalume) (20 lm/0.79 mils)

 Corrosion–erosion of the base steel and coating in edges because of direct action of the winds  Corrosion of the steel base and coating at the scribe  Damage in 75%

Lmax: the maximum length affected from the edge (of the coating and/or base steel) exposed to the direct action of the preferential winds.

Table 6 Metallic-coating thickness measurements by metallography Coating

Non-continuous hot dipping galvanizing Continuous hot dipping galvanizing Continuous hot dipping Zn/5Al Continuous hot dipping Al/13Si Flame spraying Zn/15Al Flame spraying Al

Initial thickness (lm)

Thickness measurement (lm)

Difference in thickness (lm)

70

46

14

30

42

18

50

40

10

40

38

2

150 150

144 139

4 11

Besides the above-mentioned coatings, a continuous hot dipping zinc–aluminum-alloy-based coating (Zn/ 5Al /25 lm (1 mils)) was tested to evaluate the protective properties aluminum confers on zinc-based coatings. For the first year’s exposure, this coating performs better than the galvanized coatings (Table 4). At the end of 24 months’ exposure, however (Fig. 1), the coating surface is 100% corroded, with wind-induced peeling along the edges, reaching a maximum of 2 cm (0.79 in.). Corrosion was observed on 22% of the base steel surface, with better performance than continuous hot dipping galvanizing (100%), but not when compared with non-continuous hot dipping galvanizing (7%). This is due to the greater thickness of the latter 60 lm (2.36 mils) vs. 25 lm (1 mils) for the Al/Zn coating), and also to the Zn/Fe alloy layers present thereon. Of the zinc-based coatings, the best performance was given by the flame spraying Zn/15Al and pore sealer (wash primer) coating even better than the one has vinyl sealer. There was corrosion only in the scribe coating during the first 24 months’ exposure, as well as sand and salty compounds accumulated in the scribe. It is worth mentioning that the flame spraying Zn/15Al and pore sealer (wash primer) coating was one of the thickest (70–130 lm/2.76– 5.12 mils), which makes it highly resistant to the conditions predominating at La Voz Station. In this atmosphere, therefore, the performance sequence for zinc-based coatings is as follows:

Fig. 1. Performance of hot-dip galvanizing (continuous and non-continuous) compared with continuous hot dipping Zn /5Al after 24 months’ exposure. Note the wind effect on the left edges.

Flame spraying Zn/15Al and wash primer (70–130 lm/2.76– 5.12 mils) > flame spraying Zn/15Al and vinyl sealer (150 lm/ 5.91 mils) > non-continuous hot dipping galvanizing (60 lm/ 2.36 mils) > continuous hot dipping Zn/5Al (50 lm/1.97 mils) alloy > continuous hot dipping galvanizing (20 lm/0.79 mils). Fig. 2 presents the visual aspect of zinc-based coatings after 24 months’ exposure, where it can be seen that the best-performing coating is the flame spraying Zn/15Al and pore sealer (wash primer) coating. In general, the performance of the aluminum-based coatings (Table 5) under the aggressive conditions at La Voz Station was better than that of the zinc-based coatings (see Fig. 3). The worst performance was given by the continuous hot dipping 55Al/Zn (Galvalume), the least thick of the aluminum-based coatings (20 lm/0.79 mils). After 6 months’ exposure, wind

O. de Rincón et al. / Construction and Building Materials 23 (2009) 1465–1471

1469

Fig. 2. Performance of all zinc-based coatings after 24 months’ exposure. Note the excellent performance of the flame spraying Zn/15Al and pore sealer (wash primer) coating and the wind effect on the left edges.

Fig. 3. Performance of continuous hot dipping Al/13Si coating after 24 months’ exposure. Note the wind effect on the left edges.

induced corrosion over 70% of the coating surface was visible at 10 (including edges), as well as severe corrosion of both coating and base steel at the scribe. After 24 months, there was corrosion–erosion in both coating and base steel on the left edge, with a maximum reach of 3 cm (1.18 in.) and 1.5 cm (0.59 in.), respec-

tively; with corrosion over 33% of the coating surface and 8% of the base steel. Additionally, there was base steel corrosion over 100% of the scribe, with coating corrosion with a maximum reach of 2 cm (0.79 in.) from the scribe. This coating suffers severe corrosion in the zinc-rich phase of the atmosphere predominating at La Voz, classified as > C5. The release of aluminum-rich dendrite particles was determined in the neighborhood of points where the substrate (steel) is exposed. High wind velocity significantly speeds up coating deterioration, most likely breaking off the dendrites in the exposed plates. Due to its thinness (40 lm), the cathodic protection provided by this coating is very short lived in such an aggressive medium. Fig. 4 shows a microphotograph of one of these probes exposed for 42 months at La Voz station. Note the intense coating corrosion with aluminum-rich dendrite loosening, and the severe substrate corrosion caused by high salinity. It is well known that this coating corrosion is produced by the zinc-rich interdendritic phase. Its performance is strongly affected by the microstructure of the alloy since the greater and narrower circuits formed by the zinc-rich phase enhance its performance, because the corrosion products formed by the zinc-rich layer will be more easily trapped within these circuits. This trapping diminishes the spread of corrosion and would therefore provide better coating performance. However, these circuits are not very narrow in the coating used in this study and they are unable to trap corrosion products, thus leading to poor coating performance.

Fig. 4. Micrograph (SEM) non-continuous hot dipping galvanizing (a) and continuous hot dipping Al/13Si (b) coatings after 12 months’ exposure. Note the galvanic effect in (a).

1470

O. de Rincón et al. / Construction and Building Materials 23 (2009) 1465–1471

The continuous hot dipping Al/13Si (of thickness greater than 40 lm (1.57 mils)) failed after 6 months’ exposure, with corrosion-product formation on the coating at the scribe and localized incipient corrosion–erosion of the coating edges directly produced by wind action. After 24 months’ exposure, this coating showed pitting corrosion over 100% of the surface, with corrosion product formation on the base steel at localized points on the scribe and edges, due to direct wind action. There is also corrosion in the coating at the scribe, with a maximum reach of 2.2 cm (0.87 in.) (galvanic action of the coating). With time, however, its performance was better than that of the continuous hot dipping 55Al/Zn (possibly because of greater thickness) and that of the galvanized coatings. Furthermore, the galvanic protection of the substrate afforded by the Al-based coatings at the scribe was better. The performance of the flame spraying Al coating, with its greater thickness 150 lm (5.91 mils), is excellent after 12 months’ exposure, with localized corrosion in the base steel and corrosion on the left edge of the coating, caused by direct wind action (Table 5). Evaluation after 24 months reveals 100% corrosion in the coating at the scribe, with multiple pitting on the surface, as well as localized corrosion in the base steel at the scribe and localized coating corrosion on the lower, upper and left edges. Better performance was given by the flame spraying Zn/15Al (Table 4), with the same thickness (150 lm (5.91 mils)) as the flame spraying Al coating. The first failure of this coating occurred after 12 months’ exposure, with corrosion-product formation on the coating and base steel at the scribe visible at 10, as well as coating corrosion at the edges produced by direct action of the prevailing winds at the station. After 24 months’ exposure, there is corrosion over 30% of the coating surface and the base metal at the edges due to wind action. It is important to point out that the scribe did not reach the base steel in these probes, so it is impossible to observe the galvanic effect of the coating when there is a defect. Comparison of the flame spraying aluminum with the Zn/15Al (sealer with vinyl), both with like thickness of 150 lm (5.91 mils), revealed the beneficial effect of the aluminum/zinc mixture in the coating, both from the corrosion standpoint and erosion resistance at the edges. This is due to the combined barrier (Al) and galvanic (Al/Zn) effect that affords the coating better protective properties in this highly chloride contaminated, humid (RH > 90%) and erosive environment. This effect is also found when comparing the uniform corrosion of continuous hot dipping 55Al/Zn (20 lm) with the continuous hot dipping Zn/5Al (25 lm) and continuous hot dipping galvanizing (20 lm), with like thickness, where the former had better behavior (Fig. 4). Like zinc-based coatings, thickness is the determining factor for aluminum-based coating lifetime, regardless of the application method. It is established that for the atmosphere at La Voz Station the sequence for aluminum-based coatings as a function of their performance is as follows: Flame spraying Zn/15Al and wash primer (70–130 lm/2.76– 5.12 mils) > flame spraying Zn/15Al and vinyl sealer (150 lm/ 5.91 mils) > flame spraying Al (150 lm/5.91 mils) > continuous hot dipping Al/13Si (40 lm/1.58 mils) > continuous hot dipping 55Al/Zn (20 lm/0.79 mils). However, it was the flame spraying Zn/15Al and pore sealer (wash primer) coating that gave the best general performance among all the coatings tested at this station. Fig. 5 shows the progressive increase in length of damage at the left edges, as produced by the prevailing winds (with speeds of up to 7 m/s/22.96 ft/s and gusts of 9 m/s/29.53 ft/s) after each evaluation. Fig. 6 shows the percentage of metallic coating and base metal area corroded after 24 months’ exposure. It can be seen that the percentage of corroded area is greater for zinc-based coatings

Fig. 5. Graphic representation of length of corrosion from edge exposed to the prevailing winds.

Fig. 6. Graphic representation of the percentage of corroded area in metallic coatings.

(continuous hot dipping Zn/5Al) than for aluminum-based coatings (continuous hot dipping 55Al/Zn). This is because aluminum-based coatings afford greater resistance to the aggressive medium. It is important to point out that the corroded area was measured discarding 1 cm (0.39 in.) from the edge so as to avoid considering the corrosion–erosion effect. The smallest percentage of corroded area was obtained with the continuous hot dipping 55Al/Zn, with 33% corroded area for the coating and 8% for the base steel after 24 months’ exposure. 3.3. Coating-thickness loss Coating-thickness measurement for weight loss was determined only for zinc-based coatings because there is localized corrosion (pitting) in aluminum-based coatings and these are therefore of little value for this type of evaluation. Fig. 7 shows average annual coating-thickness loss rate (TLR) for the zinc-based probes tested for 24 months. The effect of the aluminum in the coating is again observed, where the smallest TLR (9.27 lm/0.36 mils) was obtained for the Al/Zn-based coating. There was not much of a difference in the case of the galvanized coatings (18.43 lm/year/0.73 mils vs. 17.7 lm/ year/0.70 mils). It was different for erosion, however, which was greater for the continuous hot dipping galvanizing (1.25 cm/ 0.49 in.) for steel and coating. As already indicated, this is due to

O. de Rincón et al. / Construction and Building Materials 23 (2009) 1465–1471

1471

this type of coating, because of little thickness 5 lm (0.20 mils), shows no resistance to this very aggressive atmosphere. 3. The aluminum/zinc mixture in the coating produces a beneficial effect from the standpoint of both corrosion and erosion resistance at the edges. 4. Zn/Fe alloy layers in the galvanized coatings provide greater resistance to the aggressive environment, where the erosive effect of the particle-laden wind plays an important role. Acknowledgement Fig. 7. Average yearly coating-thickness loss rate (TLR/lm) for zinc-based metallic coatings exposed at La Voz Station.

the presence of Fe/Zn in the layers of alloy in the non-continuous galvanizing process. Here the coating-thickness effect can be seen in that, even though the Al/Zn coating has lost less thickness with time, 22% of the base steel area under it is exposed, which is greater than that of the galvanized coating (7%). 4. Conclusions 1. Among the metallic coatings on carbon steel tested, the best performance was given by the flame spraying Zn/15Al (wash primer) coating. 2. The worst performance among the metallic coatings on carbon steel tested was given by electroplating (Zn), with total coating loss before 6 months’ exposure, thus showing that

The authors thank Condes (Consejo de Desarrollo Científico y Humanístico) Universidad del Zulia, Venezuela, for the economic support. References [1] Feliu S, Morcillo M. Mapas de España de Corrosividad Atmosférica. CYTED. Madrid, España; 1993. [2] Uhlig H. Corrosión y Control de Corrosión. New York, USA: John Wiley & Sons Inc.; 1985. [3] Uhlig H. The corrosion handbook. New York, USA: John Wiley & Sons Inc.; 1978. [4] P. Furtado, Introducción a la Corrosión y Protección de las superficies Metálicas. Belo Horizonte, Brasil: Imprensa Universitaria da UFMG; 1981. p. 135, 235– 247. [5] Munger CH. Corrosion prevention by protective coating. Houston (TX): NACE Publications; 1984. [6] Uller L, Morcillo M. Proceedings of the 11th international corrosion congress, vol. 2, AIM, Milan, Italy; 1990. [7] de Rincón OT, Rincón A, de Romero M, Sánchez M, Prato M, Fernández M [A new vision of corrosivity mapping]. Mater Perf NACE Int 1998;37(12):48–53.