6.2.3 Analysis methods of degradation processes

6.2.3.1 Microscopic analysis

Light microscopy can be useful to get a general overview on the appearing degradation mechanisms. In the case of transparent coatings, the degradation on both the transparent top coating and the nontransparent coating underneath can be examined by moving the sample in z-direction towards the microscope objective. As Fig. 6.3 shows, the degradation on the glass surface and corrosion in the silver layer underneath can be made visible by adjusting the focus.

Confocal microscopy allows measuring the depth of pinholes in paint layers, pitting corrosion craters or erosion effects (see Fig. 6.3: glass erosion) by analyzing the obtained three-dimensional images.

Analysis with the scanning electron microscope (SEM) has the advantage of achieving a higher resolution compared to light microscopy. However, optically transparent coatings are not transparent to the scanning electron beam, therefore e.g., the silver layer underneath the glass cannot be examined unless a cross section of the sample is prepared. Special care must be taken when preparing cross sections through degraded areas to avoid additional damage due to the sample preparation tool. Glass samples may be prepared with a glass cutter and breaking them. With this method it is difficult to reach the area of interest where the degradation occurred (often only in the range of a few microns). The sample may then be polished carefully until a convenient cross section through the degradation area is reached (see Fig. 6.4).

A further improvement in resolution can be achieved with transmission electron microscopy (TEM), which typically has a resolution of <1 nm. This technique is suited to the analysis of materials with thin coatings in the nanometer scale (e.g., the physical vapor deposition (PVD) coating stack of enhanced aluminum reflectors, see Fig. 6.5). The thickness of the samples to be viewed in TEM need to be smaller than 100 nm and the preparation can be a complex procedure. Focused ion beam is a technique suited for sample preparation of aluminum reflectors, introducing much less additional degradation than other sample preparation methods like polishing.

SEM and TEM are often combined with energy-dispersive X-ray spectroscopy (EDX). It is possible to perform elemental analysis of the sample at a narrow spot of interest, along a line (line-scan), or by creating maps (EDX-mapping, see Fig. 6.6). This technique helps to identify the chemical composition of coating stacks or to detect eventual environmental contaminants. In Fig. 6.6, EDX-mapping is performed at the cross section of a local silver corrosion spot. It was identified that the remaining silver reacted to silver chloride and that the copper layer diffused into the prime coating.

6.2.3.2 Optical analysis

Optical analysis is performed to measure the actual performance loss (e.g., changes in reflectance for a solar mirror material) over the exposure time. The samples are periodically measured. The measurement frequency for outdoor exposed samples is typically 6 months. Fig. 6.7 shows the measured annual degradation rate in solar-weighted hemispherical reflectance △ρ_s(300–2500 nm, 8 degrees, h)=0.2%/year by NREL [11]. This parameter is usually measured with double beam spectrophotometers with an integrating sphere of diameter ≥150 mm.

In addition to hemispherical measurements, the specular reflectance shall be measured at defined acceptance angles in the range of φ=[3,20]mrad. Commercial reflectometers do not allow for spectral measurements at the current state of the art. Monochromatic measurements are performed at near-normal incidence (incidence angle θ_i≤15 degrees). Usually, the measurement spot is no larger than 10 mm in diameter. If possible, the measurements should be performed for several wavelengths. If measurements at larger incidence angles are to be taken, then the reflectance needs to be measured separately for s- and p-polarized light, and the final result consists of the average of both measurements. The most frequent incidence angles regarded over the timeframe of an entire year at parabolic trough and solar tower plants is between 30 and 40 degrees. Therefore, measurement at off-normal incident angles is encouraged.

Especially when characterizing samples with localized degradation effects or inhomogeneous behavior across the surface (see Fig. 6.8), several measurements should be taken for statistical reasons. It is recommended to measure always at the exact same spots throughout the exposure test for proper monitoring of the optical properties. A mask, where the measurement position of the reflectometer is marked, can help to reallocate the correct measurement spot. The measurements should be taken with sufficient distance from the sample edge to prevent measuring edge degradation effects. It is recommended to take at least five specular and three hemispherical reflectance measurements on a 10×10 cm² sample.

6.3.1.1 Damp heat test IEC 62108 (10.7a, b), or IEC 61215 (10.13)

The damp heat test is used to test the resistance against penetration of humidity through the paint coatings at relatively high temperatures. There are two options to perform the test. In both cases, the samples are subjected to a constant climate of 85±5% relative humidity in a climatic chamber. In Test 10.7a, the operating temperature is 85±2°C and the testing time is 1000 h. Test 10.7b operates at a lower temperature of 65±2°C, but the testing time is higher (2000 h) [15]. Test 10.7a is similar to Test 10.13 from IEC 61215 [16].

6.3.1.2 Condensation test ISO 6270-2

The resistance to condensation water is tested by exposing the samples to constant conditions of 40±3°C with 100% relative humidity provoking condensation on the sample surface [17]. Several climatic chambers are available on the market to perform this test.

6.3.1.3 UV and humidity test ISO 16474-3

The UV and humidity test according to the standard ISO 16474-3 [18] replaces the previously commonly used ISO 11507. It consists of the following cycle: in the beginning, samples are exposed during 4 h at 60±3°C to UV-radiation in a specific testing chamber (see Fig. 6.9A). For solar reflectors, the lamp type 1A (UVA-340) is recommended. It emits radiation in the wavelength-range of 290–400 nm with a peak emission at 340 nm. The lamp intensity at 340 nm is 0.83 W/m²/nm. The total intensity in the entire lamp spectrum of 290–400 nm is around 45 W/m². Fig. 6.10 shows a typical spectrum of the UVA-340 lamp compared to daylight solar spectral irradiance according to Ref. [20]. After the radiation phase of 4 h, the samples are exposed during 4 h at 50±3°C to condensation (100% relative humidity without irradiation). The total duration of one cycle is 8 h (see Fig. 6.9B). For second surface mirrors with protective lacquers from the back side (such as silvered-glass mirrors), it is recommended to expose both the front and back side of the sample towards the interior of the testing chamber, beginning with the exposure of the glass side towards the UV-lamps for at least 1000 h, and then turning the sample around to the back side for at least 1000 h, meaning the total testing time in this case is at least 2000 h. First surface mirrors (such as silvered-polymer films or aluminum reflectors) should only be tested with the front side facing the interior of the chamber, since the backside will be protected in operation by a substrate material. The commonly performed testing time is at least 1000 h.

6.3.1.4 Neutral salt spray (NSS) test ISO 9227

The NSS test is a widely used corrosion test to simulate coastal environments. Samples are exposed to constant conditions of 35±2°C with a spray of a NaCl solution (50±5 g/L, pH 6.5–7.2) at 100% humidity. The samples should be positioned with a tilt angle of 20±5 degrees respect to the vertical within the salt spray chamber (see Fig. 6.11). The amount of the sprayed solution should be adjusted to obtain a condensation rate of 1.5±0.5 mL/h on a surface of 80 cm² [21]. The condensation rates should be checked at various locations within the chamber to ensure homogeneous distribution of the salt fog. A circular chamber design usually leads to higher reproducibility of test results of samples tested in different positions within the chamber.

6.3.1.5 Copper accelerated salt spray (CASS) test ISO 9227

Further acceleration compared to NSS can be achieved with the CASS test. Samples are exposed to 50±2°C and 100% relative humidity with a constant spray of demineralized water containing 50±5 g/L NaCl. Additionally, the solution contains 0.26±0.02 g/L of copper chloride (CuCl₂). The pH of the sprayed solution lies between 3.1 and 3.3, and is adjusted using hydrochloric acid (HCl), sodium hydroxide (NaOH), or sodium bicarbonate (NaHCO₃) [21]. The salt spray chamber can be used for both NSS and CASS. However, once a CASS test has been performed within the chamber, it is not recommended to perform later NSS experiments due to the chemical contamination of the chamber interior, as this will lead to modified test results. The commonly performed testing time is 120 h.

6.3.1.6 Kesternich test DIN 50018 or ISO 6988

The Kesternich test is used to simulate aggressive industrially polluted environments. During the first 8 h of this cyclic test, the samples are exposed in a climatic chamber to 40±3°C and 100% relative humidity. Additionally, in the DIN 50018 [22] standard, 0.33% or 0.67% of the volume of the testing chamber is charged with sulfur dioxide gas (SO₂). During the following dry phase of 16 h, the samples are exposed to the ambient conditions of temperature and humidity. The duration of one cycle is 24 h (see Fig. 6.12). The testing conditions in ISO 6988 [23] are identical, except that the sulfur dioxide gas concentration is reduced to 0.067% of the volume of the testing chamber.

6.3.1.7 Thermal cycling test IEC 61215 (10.11) or IEC 62108 (10.6)

The resistance to formation of cracks or delamination due to thermal influence is tested with thermal cycling tests. In IEC 62108, Test 10.6 (Option TCA 3) [15], the samples are subjected to 2000 cycles from −40°C to 65°C. A dwell time of at least 10 min within ±3°C of the high and low temperatures is required (see Fig. 6.13A) [15]. The procedure defined in IEC 62108, Test 10.6 (Option TCA 1) works in the exact same manner, but the maximum temperature is increased up to 85°C (see Fig. 6.13B). The number of cycles is 1000. The thermal cycling test defined in IEC 61215, Test 10.11 is identical to IEC 62108 Test 10.6 (Option TCA 1), but the cycle number is reduced to 200 cycles [16] and the heating and cooling rates are limited to a maximum of 100°C/h. The cycling frequency in IEC 62108 is defined between 10 and 18 cycles per day.

6.3.1.8 Combined thermal cycling and humidity test

The following test combines thermal cycling and exposure to humidity. This seems reasonable because gradually cracks appearing in the paint system due to different thermal expansion coefficients of the layers will allow the penetration of humidity during the condensation phase and eventually lead to corrosion. The test is currently under standardization in the Spanish AENOR (AEN/CTN 206/SC 117/GT 2 “Components of solar thermal plants”) standardization committee [24]. The test consists of a 4-h phase at 85°C, followed by 4 h at −40°C. Three options are possible to perform the following humidity cycle. Method A defines the same conditions as in to ISO 6270-2, with 40°C and 97±3% relative humidity (see Section 6.3.1.2, Fig. 6.14). Method B consists of the damp heat test (see Section 6.3.1.1), for which two options are given: either at 85°C or at 65°C with increased exposure time of 40 h. Tables 6.1–6.3 summarize the operating conditions of this test.

6.3.1.9 Humidity freeze test IEC 62108 (10.8)

A similar test to the one described in Section 6.3.1.8 is the humidity freeze test. It is used to check the coating resistance to formation of cracks or delamination due to thermal influence in combination with humidity. In difference to the previously described test, the humidity freeze is performed in two steps. At first, the samples are subjected to 400 cycles from −40°C to 65°C. A dwell time of at least 10 min within ±3°C of the high and low temperatures is required (see Fig. 6.15A and Section 6.3.1.7). In the second step, the samples are subjected 40 times to the humidity freeze cycle shown in Fig. 6.15B [15]. The total duration of the test is approximately 1500 h.

6.3.1.10 Abrasion and erosion tests

We differentiate between abrasion effects, caused by occasional contact with other media such as pressurized water or brushes for cleaning, and erosion effects, caused naturally by the environment, e.g., by windblown particles.

Several standards are used to test the abrasion resistance of paints and varnishes. For example, ISO 11998 defines a procedure to determine the wet-scrub resistance and cleanability of coatings [25]. Alternative accelerated abrasion tests involve testing with the Taber Abrasor scrub resistance tester or a similar device, according to ISO 9211-4, in which the surface is exposed to repetitive wiping with a defined abrader [26]. The normal force of the abrader on the sample can be controlled and modified, which leads to more reproducible results compared to other abrasion tests.

MIL-STD-810G defines a dust and sand test to determine the resistance against particle erosion in desert regions [27]. Such tests need to be carried out in elaborated sandstorm-simulating chambers able to control wind velocities and particle concentrations. The tests defined in MIL-STD-810G and also in other available standards like ASTM-D968-05 or ASTM-F1864-05 are too aggressive for reflective materials and are not suited for CSP applications [28,29]. Advanced standards with modified testing conditions are required.

Recent preliminary analysis has identified that more reasonable parameters to simulate the effect of sandstorms can be as follows: Arizona test dust according to ISO 12103-1 A4 coarse with a particle diameter range of 1–180 μm [30], constant wind velocity of 12.5 m/s, normal impact angle, constant particle concentration of 100 mg/m³, testing time of 10 min. Five cycles can be performed, while the changes of the optical properties should be measured after each cycle. Those parameters have been selected based on the available meteorological data at Moroccan desert sites. The research on how to correlate these parameters with different outdoor exposure sites is still ongoing (see Section 6.4.3).

A simpler sand erosion test is defined in DIN 52348 [31]. It consists of a sand trickling method, where particles acted by gravity from a height of 1.65 m impinge on a rotating sample. This method achieves small impact velocities (2.5–4.0 m/s) and uses larger particles (0.5–0.71 mm). Therefore this test reproduces the natural effects only to a certain extent. However it is easy to perform and highly reproducible, and for this reason it is useful to compare the erosion resistance of different materials.

6.3.1.11 Corrosive gases tests

Some gases present in the ambient as trace substances may promote and accelerate the corrosion process of most metallic materials. The main gases that have been pointed out as corrosive pollutants are chlorides, sulfates, nitrates, and hydroxides. These substances are detected with noteworthy concentrations in the ambient at those areas close to industries and cities. Several tests may be conducted to simulate and enhance the influence of corrosion-promoting gases:

• ISO 21207 [32] is an accelerated corrosion test used in assessing the corrosion resistance of products with metals (e.g., electronic components) in environments where there is a significant influence of chloride ions and of corrosion promoting gases from industrial or traffic air pollution. The test involves alternate exposure to NO₂ and SO₂ at 95±3% RH and 25±1°C, and neutral salt spray test, according to ISO 9227. Method A simulates moderately aggressive traffic, while Method B simulates a more severe traffic or industrial environment.

• ISO 10062 [33] is intended to determine the influence of one or more pollutant gases at very low concentrations on metals and alloys with and without protection against corrosion, under specific conditions of temperature and humidity. Six different test methods are covered, in which two temperature and humidity levels (which could be 40±1°C/80±5% RH or 25±1°C/75±3% RH) and several combinations of NO₂, SO₂, Cl₂, and H₂S are possible.

• EN 60068-2-42 [34] is a test of SO₂ for contacts and connections or for precious metal compounds, excluding silver. The test is done under 25±2°C/75±5% RH, with 25 ppm of SO₂.

• EN 60068-2-43 [35] is similar to the previous test, but with H₂S as the promoting corrosion gas. The goal is to check the effect of the gas on silver and silver alloys used in contacts and connections. The test is done under 25±2°C/75±5% RH, with 10–15 ppm of H₂S.

• EN 60068-2-60 [36] is a corrosion test to determine the influence of corrosive ambient on electrotechnical components, devices, and materials. Four methods are possible, combining temperature (25±1°C or 30±1°C) and humidity (70±3°C or 75±3 RH) and different gases mixtures among NO₂, SO₂, Cl₂, and H₂S.

6.3.1.12 Testing programs

Usually several samples of the same material type (and preferably of the same manufacturing batch) are tested simultaneously in accelerated aging testing programs to check their stability against a variety of possible environmental stresses. Table 6.4 shows a “basic” testing program, which can be used for initial screening of a material's durability. The basic program is a list of minimum requirements that the material needs to withstand for potential use outdoors. Solar mirrors, regardless of their type, should not show degradation effects larger than 200 μm and reflectance losses in any test of the basic program.

The “advanced” testing program consists of similar tests, with the difference that the testing times are highly increased. The total duration of this testing campaign including sample analysis is up to 7 months. The main purpose of the advanced testing program is to compare different materials under different stresses, e.g., by evaluating which of the tested materials fails first under certain conditions.

6.4.3.1 Field data about sandstorms

Sandstorms and duststorms should be distinguished as two separate categories. This differentiation is connected to the size of the particles involved in the event. The term dust storm is usually related to winds carrying huge quantities of loose soil and dust particles (d_mean<10 μm) to air layers some hundred meters high above the ground. The visibility can be reduced below 100 m and the particles can travel more than 1000 km [39,40].

Sandstorms, on the other hand, contain particles bigger than 10 μm that move in a low-flying cloud with a clearly marked upper surface usually not higher than 2 m above the ground [41]. Since neither phenomenon occurs exclusively, in the following text the term sandstorm addresses wind-blown particles in the air.

Aeolian particles can lead to serious effects on health, traffic, and communication systems. Furthermore, negative influences on the agriculture due to soil loss, coverage of important structures with significant amounts of sand, and the mechanical erosion of constructions are the consequence. For more details and examples on the natural and socioeconomic effects of these weather phenomena, see Refs. [39,42–45].

As stated in Ref. [46], sandstorms are expected to increase in number and severity over the next decades. Therefore there is a growing demand to understand the cause and the characteristics of such events. Especially for CSP plant operators, the early detection of sandstorm events and possible protection from them is a critical factor to assure the efficient plant performance over a lifetime range of 20–25 years [47]. Erosion-sensitive CSP components are in particular optical parts that play a role in the solar concentration process, e.g., absorber tubes and reflectors. However, the standards available are not suited to predict the optical degradation of these components during such natural events. One reason for the so far unsuccessful simulation in the laboratory is the insufficient data about physical parameters during natural sandstorms. The wind velocity v, the sand concentration c, and the sand properties (particle size distribution (PSD), particle hardness, and roundness) during these events are crucial factors [48].

There are many works dealing with the topic of sandstorms from various regions in the world with most of them from China, being a country severely plagued by desertification. From the literature review it is quite obvious that sandstorms exhibit very different behavior in the various arid or semiarid areas of the world (see Table 6.6).

To achieve a realistic erosion simulation of the components of a CSP plant, a field campaign at the specific site of the plant is recommended. The wind velocity at respective heights above the floor is the least problematic parameter to determine.

Furthermore, studies carried out by Schütz and Jaenicke [57] and d'Almeida and Schütz [58] demonstrate that the particle properties of Aeolian sand and the sand samples taken from the soil exhibit very coinciding characteristics. However, the direct sampling of wind-blown sand is necessary to determine reliably the sand concentration at a certain height. Most commonly, passive sand traps like the big spring number eight—BSNE—(see Fig. 6.24A), a rotating wedge-shaped chamber developed by Fryrear in 1986 [48], are used. They have a major disadvantage in that no in situ values can be acquired and their sampling efficiency is not always known at the distinct outdoor conditions. The sampling efficiency is a value that stands for the percentage of particles that are actually collected within the sampler opening area/volume. For passive systems, this value is highly dependent on wind velocities and particle characteristics.

Therefore it is advisable to use an active dust sampler. These devices are equipped with a high-volume pump and sample a precise air volume. The suspended particles are stored on filter disks that can be evaluated later on in the laboratory in terms of their above-mentioned properties. A general construction scheme is shown in Fig. 6.24B (see also Fig. 6.2C). With these devices it is possible to record only specific time intervals of interest. By changing the sample heads properties, e.g., by applying a PM10 filter head, it is possible to collect particles up to a certain diameter only. Additionally, the defined blower volume flow allows the deducing of the mean concentration of particles in the air during the event.

The sand collected with active or passive samplers needs to be investigated in the laboratory in terms of its above-mentioned properties.

6.4.3.2 Accelerated erosion setups for laboratory experiments

Having acquired all the important parameters by field measurements, the natural erosion process can be simulated under controlled laboratory conditions. The current state-of-the-art approaches can be subdivided into three different methods:

• sand trickling [59,60];

• sand blasting or open loop wind tunnel [61,62]; and

• closed loop wind tunnel [63].

Each one offers advantages and disadvantages. The trickling setup is quite easy to operate, but is limited in the applied particle velocity, since the test sand is only accelerated due to the force of gravitation. While the sand trickling and open loop wind tunnel setups facilitate very good control mechanisms in terms of particle concentration, the closed loop wind tunnel does not consume that much test sand material. Yet, since no standard is developed, research is going on with all these three devices. The overall results from literature review in this topic are as follows:

• The loss of specular reflectivity increases with the wind velocity [62,63].

• Longer test times or higher sand concentrations lead to a linear increase in reflectance loss [59,60,64].

• Larger grain sizes and harder sands also promote the erosion severity [61].

Furthermore, to give a clearer impression on the effects on mirrors and the capability of erosion setups already in operation mode, an aluminum reflector is shown in Fig. 6.25 after natural and artificial erosion. The sand trickling test has been stopped when the same reflectivity decrease of the mirror as in the outdoor sample was reached. In the microscope images at the bottom of Fig. 6.25, it can be observed that the PSD of the laboratory tests does not completely correspond to the outdoor distribution (see also Fig. 6.21). However, the reflectance decrease due to particle erosion during 20 months of outdoor aging could be simulated by a laboratory treatment that only lasted some seconds. Additionally, focusing on the damage of one large particle in both outdoor and laboratory tests (see Fig. 6.25 top), similar damage is seen.

In order to improve the achieved erosion pattern towards a more realistic appearance, further research is being carried out. One approach is to combine different erosion tests in a testing sequence. For example, as a first step, the sample could be tested in the closed or open loop wind tunnel. Here small particles in combination with high wind velocities reproduce the numerous small impacts detected outdoors. In a second step, the same sample could be tested using the sand trickling method to reproduce the larger impacts.

Additional research activities are accelerated erosion testing with different sand types and further field data acquisition of real sandstorm events. An accelerated erosion guideline to simulate relevant North African exposure sites is expected to be published in 2017.