The influence of the composition of stainless steels on atmospheric corrosion resistance in a marine environment in Dubai was investigated after 2 y and 4 y of exposure. Different stainless steel grades with different surface finishes were included in the investigation: three ferritic stainless steels, five austenitic stainless steels, and four duplex stainless steels. The alloying elements chromium (Cr) and molybdenum (Mo) both had a beneficial influence on the corrosion resistance. The pitting resistance equivalent number (PREN=%Cr + 3.3%Mo + 16%N) and the (%Cr + 3.3%Mo) content in the surface film correlated well to the atmospheric corrosion resistance. An increased Cr content both in the bulk material and in the passive film improved the atmospheric corrosion resistance and the additional presence of Mo was effective in preventing red rust and also reduced the depth of pits. The depth of the pitting attack and the degree of aesthetic degradation were both influenced by alloying level, surface finish, and exposure conditions (open and sheltered). In the severe marine environment in Dubai, it is necessary to use Mo-bearing high-Cr stainless steel for adequate atmospheric corrosion resistance. The most resistant stainless steel grades were the high alloyed grades which ranked in the order UNS S31254 ∼ UNS S3750 < UNS S34565 < UNS S32654. The duplex stainless steel grade S32205 may be considered for construction and architectural materials in Dubai but is likely to require more maintenance.
The use of stainless steel in structural and architectural applications has increased significantly over the last 15 y. Stainless steel is selected for reasons of aesthetics, corrosion resistance, long-term durability, mechanical strength, and a combination of these factors. When selecting stainless steels in structural applications, the most important reason is a high corrosion resistance to ensure long-term durability. The alloying composition is one of the most important factors that affect the corrosion resistance of stainless steel. The alloying elements chromium (Cr), nitrogen (N), and molybdenum (Mo) in the bulk material often have the largest impact on the corrosion properties.1-3 Previous studies have indicated that the alloying levels in terms of pitting resistance equivalent number, PREN (PREN=%Cr + 3.3%Mo + 16%N) has proved to be a good predictor for the resistance in chloride environments and atmospheric corrosion.2 The higher the PREN, the higher the resistance to pitting corrosion. The most important issue when selecting stainless steel in architectural applications is the aesthetic degradation. Here the alloying content in the surface film is primarily responsible for its resistance to degradation,1,4 although the efficiency of the protective oxide film also depends on the surface finish. Chromium has been reported to be the most beneficial alloying element in terms of resistance to aesthetic degradation because it promotes a high stability of the passive film.4-6 Other alloying elements in the surface film such as Mo and Si also have an influence on atmospheric resistance.3-4,9-12 The surface finish influences the degradation resistance of stainless steel because it can affect the film formation process and film structure, even though the alloy levels in the bulk material are the same,4,7 in addition to affecting the adherence of extraneous contamination. Surface treatments such as acid picking can also result in increased chromium content in the passive film.2,4,9
For selecting stainless steel for structural and architectural applications, there are several guidelines and standards available, such as EN 1993-1-4 Annex A (Eurocode 3) and the IMOA Site and Design Evaluation System tool (IMAO program).13-14 The EN 1993-1-4 Annex A standard provides a procedure for selecting an appropriate grade of stainless steel for the service environment where the structural integrity is the primary concern.13 In accordance with the standard, the selection of grade depends on the environmental conditions which are assessed in terms of three components: risk of exposure to chlorides from salt water or deicing salts (distance from the sea and roads with deicing salts), risk of exposure to sulfur dioxide, and cleaning regime including washing by rain. The evaluation in the IMOA program uses five different factors: environment pollution, coastal and deicing salt exposure, local weather pattern, design considerations, and maintenance schedule. The IMOA program is a useful tool as a grade selection guide when surface appearance matters.14
The Dubai test program described in this paper was initiated because of the paucity of relevant information for a dry marine Middle-Eastern climate. The grade selection for a specific marine environment in Dubai must be adequate for the required performance. Selecting the suitable stainless steel grade requires knowledge of the actual location of the application and the atmospheric conditions. The main objective of this paper is to present information about the performance of a number of stainless steel grades at a marine site in Dubai after several years of exposure. The atmospheric corrosion resistance of stainless steel coupons was determined in terms of depth of localized attack and extent of red rust, discoloration, and staining. The corrosion performance and degree of degradation are related to the alloying element levels in the bulk material and in the surface film.
Materials and Preparation
Twelve stainless steel grades were tested as plain (sheet), welded, and creviced samples. The samples were made by cutting the stainless steel to dimension of 150 × 100 × t mm (t=thickness) and the cut edges were then dry ground (320 grit) to minimize edge attack by removing residual carbon steel from cutting and giving a smoother surface. The samples were thereafter marked and cleaned before mounting in accordance with ASTM GI-90.15 In this paper, the corrosion properties of the base material are the main concern, so the welded and creviced areas were disregarded and will be the subject of future study. The samples were exposed in open and sheltered conditions at an angle of 45° and orientated to the northwest, facing the sea. Duplicate samples were exposed for most of the steel grades. Different sets of samples were investigated for 2 y and 4 y of exposure. For the brushed surfaces, the specimens were oriented with the brushing lines horizontal. The characteristics of the materials, including the typical chemical composition, surface finish, and the PREN (%Cr + 3.3%Mo + 16%N) are given in Table 1. All steels were taken from standard commercial production and had normal microstructures with very low levels of nonmetallic inclusions.
The test site was provided by the Dubai Electricity and Water Authority (DEWA) site in Dubai and is situated at latitude of 25°02′53.9′′ North and longitude of 55°06′05.0′′ East. Exposures were performed from May 2010 to May 2014. The racks were located directly on the sea shore of the Arabian Gulf and exposed in open and sheltered conditions, see Figure 1. It has been reported that the severe marine environment in the Arabian Gulf is characterized by high temperatures, high salt levels, and low rain.2,16-17 The corrosiveness class of the test site in this study was CX in accordance with the standard ISO 9223,2,4,17 i.e., a very severe environment. Although the ISO 9223 system is intended to be generic, it is calibrated for assessing atmospheric corrosion risks for reference materials (carbon steel, zinc, copper, and aluminum). These materials suffer general corrosion in atmospheric exposures, while stainless steel is at risk of localized corrosion. As the risk factors and corrosion processes are different, it is perhaps unsurprising that a classification system intended to deal with general corrosion risk (i.e., corrosiveness classes) does not appear to be as useful for the localized corrosion of stainless steel.
Evaluation for Corrosion Performance and Appearance Rating
After exposure, all specimens were first cleaned with tap water in order to get rid of dirt and dust, followed by a short rinse in acetone. The cleaned specimens were evaluated for atmospheric corrosion resistance. There is no standardized way to characterize stainless steel coupons exposed to natural atmosphere sites. Two different criteria were therefore chosen for evaluation in this investigation.2,4 The first is the ranking of the corrosion resistance of the different surface conditions based on the extent of localized attack on the flat surfaces (number of pits and the depth of corrosion attack) under a microscope at 20× magnification. Corrosion attack shallower than 25 μm is classified as surface etching and excluded.17 The difference between pitting and etching on the surface is shown in Figures 2(a) and (b). Edge attack was disregarded because in the majority of applications these are not usually exposed. Cross sections of the most severely attacked grade, UNS S40977, were examined using scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) after 4 y of exposure, as shown in Figure 2(c). Generally, the mass loss was not found to be useful for evaluation because the edges could corrode heavily during the long exposure time. The second criterion used was a visual rating of the aesthetic degradation in terms of the extent of red rust, discoloration, and staining on the exposed surfaces. The concept of the rating number (RN) was developed by modifying the procedure described in the JIS G 0595 standard,19 which involves comparison with standard specimens. In the present case the ranking was defined in such a way that more corrosion led to a higher ranking number, the opposite of the principle used in JIS G 0595.19 The average RN was calculated from 3 values obtained from 3 different evaluators. The relationship between the RN and percentage of the specimen area with rust and staining is shown in Figure 3. The rating “9” means that the entire surface is covered by rust and stains, whereas “0” means no rust/staining or discoloration and the appearance is the same as before the exposure. The difference between discoloration, staining, and rust is that discoloration is defined as a change in the overall surface color, i.e., duller, faded, or darker. Staining is defined as a result of corrosion attack. Although it can look quite significant in terms of appearance, this corrosion usually does not penetrate into the steel, and does not affect the structural integrity. Rust is formed on the surface of stainless steel when a condition develops in which corrosion products are formed.
Characterization of Alloying Levels in Surface Film
The surface composition was measured by glow discharge optical emission spectrometry (GDOES). Prior to analysis all samples were wiped clean by paper soaked in isopropanol, dried by hot air, and finally blown with a high-pressure CO2 jet. GDOES compositional depth profiles were obtained with a LECO GDS 850A† spectrometer. Depth profiles were obtained by measuring emission intensities for constituent elements as a function of sputtering time. The quantitative relationship between the composition and thickness were estimated according to a standard procedure.20 The GDOES analysis was performed on the samples before exposure after cleaning the surface ultrasonically in acetone.
Atmospheric Corrosion Performance of Various Stainless Steel Grades
The atmospheric corrosion resistance results of the various stainless steel grades and surface finishes after 2 y and 4 y exposure are summarized in Tables 2 and 3 and images of specimens after 4 y are given in Figures 4 through 8. Tables 2 and 3 demonstrate the atmospheric results in terms of corrosion performance (maximum depth and number of pits) and appearance rating for open and sheltered conditions. The maximum depth of corrosion attack can be considered if corrosion is likely to constitute any serious risk to structural integrity or function, while the appearance is more relevant to aesthetic use of stainless steel.
In this study atmospheric results can be divided into three different categories: unacceptable (inadequate) grade/surface where the maximum pitting depth is 100 μm or higher, intermediate grade/surface where the maximum depth is below 100 μm, and finally acceptable grades/surfaces where no corrosion occurs. In this investigation two types of corrosion morphology were observed on the exposed surfaces. One was caused by uniform corrosion and the other appeared to be damage by pitting corrosion. An example of an inadequate grade is the ferritic grade UNS S40977, which has a chromium content of only 11.5%Cr and showed severe pitting which merged to give uniform corrosion after longer exposure time, as shown in Figures 2(c) and 4(a) and (b). Pitting corrosion was observed for the somewhat higher alloyed grades such as the ferritic grades UNS S43000 and UNS S44400, the standard austenitic grades UNS S30403 and UNS S31603, and the lean duplex grades UNS S32101 and UNS S32304. For these grades, the number of pits was more than 20 and a maximum pit depth was above 100 μm. In terms of corrosion performance, the effects of surface finishes, exposure duration, and alloy content could not be clearly identified, for these inadequate grades.
For the intermediate grade UNS S32205, pitting corrosion was very slight and below 100 μm in depth. The appearance rating for UNS S32205 in open conditions ranged between 3 and 6 after 4 y of exposure, as seen in Figure 8. The grades UNS S31254, UNS S34565, UNS S32750, and UNS S32654 were all acceptable materials and exhibited the highest resistance to pitting corrosion of the different stainless steels investigated. The specimens exposed in the sheltered conditions were less degraded than those from open conditions for UNS S32205, UNS S31254, UNS S34565, UNS S32750, and UNS S32654, while more variable trends were seen for the lower alloyed grades.
As seen in Tables 2 and 3, the maximum pit depth for the low alloyed stainless steel grades decreased with increasing alloying levels, and was larger for the longer exposure time of 4 y. It was also higher for the sheltered exposures. This may be because maintaining the passive film is more difficult for the lower alloyed grades in the presence of the higher chloride deposition observed in sheltered condition,2,4 and also because the beneficial effect of rain washing seen in more temperate environments is largely absent in the Dubai climate. Some samples had smaller pit depths at the longer exposure because of variability of samples. For the inadequate steel grades, the corrosion tended to be pitting corrosion rather than uniform corrosion and it is considered that this atmospheric corrosion was mostly caused by chloride attack from the marine environment. The results from this study showed that there is a clear threshold in terms of the alloying level of stainless steels which is required to resist the chloride-containing atmosphere in Dubai.
Three different types of degradation after 2 y and 4 y exposure were distinguished on the exposed surfaces: discoloration, staining, and red rust. The red rust and staining was more obvious for the steel grades with an inadequate alloying content, while discoloration and staining were observed on intermediate and acceptable grades. The degree of red rust, staining, and discoloration on the exposed surface is reported in terms of the appearance RN. UNS S40977 was covered with 100% red rust (RN 9) after 4 y exposure for both exposed conditions as seen in Figures 4(a) and (b). The UNS S43000, UNS S44400, UNS S30403, UNS S31603, UNS S32101, and UNS S32304 samples exhibited red rust and stains on the exposed surfaces as seen in Figures 4, 5, and 7. These figures show that there was no significant difference in appearance between these steel grades and their different surface finishes in the same exposure conditions. The spotty red rust and staining severely reduces the aesthetic appeal. For more resistant grades (higher alloying levels), discoloration and staining were the main reasons for surface degradation. The presence of pits on the duplex UNS S32205 did not cause the presence of any red rust on the exposed surface. This may be because the alloying levels such as Cr and Mo in and beneath the surface film are high enough to repassivate the pits so that no corrosion products (red rust) spread out around the pit.
The extent of red rust and staining was less for the sheltered conditions than in open conditions. This may be due to the presence of a thick protective layer of deposits (dirt/dust/particle) which is relatively benign and less night-time condensation.2,4 The surface finish also had an influence on the RN of the intermediate grade UNS S32205 in open conditions. As seen in Tables 2 and 3 there is a range of RN for UNS S32205 in open conditions. This varied between RN 2 and 5 for 2 y exposure and RN 3 and 6 for 4 y exposure. The conclusion is that in the marine environment in Dubai, a UNS S32205 with a smooth (2E-brushed) surface finish may have a satisfactory appearance. However, if a combination of a good surface appearance and corrosion performance are required, a more corrosion resistant material such as UNS S31254, UNS S34565, UNS S32750, and UNS S32654 is recommended.
Alloying Content in the Surface Film of Various Stainless Steel Grades Before Exposure
In order to understand the role of the passive film on the atmospheric corrosion behavior of stainless steels, surface analyses were performed with GDOES. Such analyses present challenges as they are close to the resolution limit of the technique, but good results were obtained with minimal startup artifacts. Carbon, which is mainly from surface contamination, was excluded from quantification, while nitrogen was present at such low levels that quantification was not considered reliable. Two different definitions of the passive film were used: either the thickness at which the oxygen level fell to half maximum, or an absolute definition of 10% oxygen. These yielded an oxide thickness which averaged approximately 3 nm or 8 nm, respectively. This is in reasonable agreement with literature data which report passive film thicknesses in the range 3 nm to 5 nm thick using other analysis techniques such as Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS).10-11,21-22 In this study the composition of the outer 5.0 nm was then selected as representative of the composition of the oxide film. Examples of GDOES analysis results, the compositional depth profiles through surface film for UNS S32654 with 2E-brushed surface, and the relative levels of chromium, molybdenum, nickel, and iron in the passive film are shown in Figure 9. A good correlation between alloying level in the bulk material and in the surface film was observed, as seen in Figure 10. This is further illustrated by the compositional depth profiles which compare different steel grades in Figure 11. Both Cr and Mo have been found in other works to increase the stability of the passive film, increasing the corrosion resistance.2-4,12 The amount of Cr in the surface film before exposure depends not only on the bulk material but also on the surface finish. Any surface treatment that enriches the Cr will effectively retard corrosion, at least in the short term, if the environment is not very severe.5-6,12
Correlation Between Alloying Element Level (PREN) and Corrosion Performance
The atmospheric corrosion resistance of stainless steel exposed in a marine atmosphere in Dubai is clearly related to the alloying content of the bulk material. The concept of PREN has been reported to be a good predictor for ranking stainless steels regarding their resistance to pitting corrosion in aqueous chloride environments. The most common PREN formula is %Cr + 3.3%Mo + 16%N.1-3 Even though the PREN value was developed for immersion conditions, it may also be relevant to the atmospheric corrosion of stainless steel in this study.
A correlation between the effects of alloying levels (PREN=%Cr + 3.3%Mo + 16%N) and the maximum pit depth is shown in Figure 12. In general, sheltered specimens were slightly less affected than the openly exposed specimens, as seen by the shallower slope of the regression lines in Figure 12. The alloying level (PREN) gave a reasonable linear fit to the pit depth data for the open conditions, with R2=0.7 after 4 y, while the correlation coefficient decreased to R2=0.5 for the sheltered condition. The true relationship is expected to be more complex, with possible incubation times and nonlinear growth, but evaluation of this requires appreciably more exposure times. The difference between sheltered and open conditions may be related to the beneficial effect of a higher proportion of sulfate in the deposits formed in the sheltered condition, as discussed in previous papers.2,4,16,23
Correlation Between Alloying Content and Appearance Rating
The appearance of stainless steel is the main concern for architectural applications. The appearance of stainless steel surface is described by several standards such as ASTM D610-01, ASTM B537-70, and JIS G 0595.2,5,24-28 However, only JIS G 0595 describes evaluation of the degree of rusting on stainless steel,19 while other standards concern the rusting that is caused by degradation of the coating on painted steel and electroplated material.25-25 The RN used in JIS G 0595 relates to the logarithm of the ratio of the percentage of rusted area to the total area of specimens surface. The estimation of the percentage of surface area rusted is performed by visual examination by comparing with the standard sample of different levels of rusting or image analysis programs.19,24-25
The evaluation of the degree of aesthetic degradation used for this study was modified from the JIS G 0595 by introducing stainless steel comparison images (Figure 3) and reversing rating scale value so that a higher value corresponds to a more degraded surface. The appearance rating included the degradation of the stainless steel surface caused by discoloration, staining, and red rust. Other standards do not consider the discoloration and staining on the surface.
The degree of aesthetic degradation for stainless steels exposed in the severe marine environment in Dubai was also related to the alloying levels of the different stainless steels. The main degradation was caused by discoloration and staining for duplex UNS S32205 or higher grades. Red rusting and staining were observed for UNS S32304 or lower grades. Although an increased Cr content improved the atmospheric corrosion resistance, a Cr increase up to 23% in the duplex stainless steel grades UNS S32304 was not sufficient to prevent red rust. The addition of Mo (coadded with Cr) was effective in preventing red rust and also reduced the depth of pits, as was seen for the duplex UNS S32205. The higher the Cr and Mo level in the bulk material, the higher their level in the surface oxide, a factor which has been recognized to increase the protective properties of the oxide film.3,12 As shown in Figures 13 and 14, there is a reasonable correlation of the appearance ratings (RN) to the alloying levels in the bulk material, also to the composition of the surface film before exposure. The PREN (%Cr + 3.3%Mo + 16%N) for the bulk composition and a simplified PREN for the surface compositions (%Cr + 3.3%Mo) both showed a good correlation to the appearance rating for shorter exposure duration (R2=∼0.7). For longer times, the bulk composition (PREN=%Cr + 3.3%Mo + 16%N) showed the strongest correlation (R2=∼0.9), indicating that the continued supply of critical elements to the surface film from the underlying metal becomes more important than the initial film composition. The increase in the RN with time reflects an increase in both the density and distribution of corrosion products.
Other factors also contribute to the difference between the tested materials. One such factor is the surface finish, as has been discussed earlier.4 In this study, all samples were tested “as-delivered from the mill” condition with various surface finishes. As a consequence, it should be borne in mind that the actual performance of any specific grade is also influenced by the actual surface finish.
A significant difference of RN was observed between the open and sheltered conditions. The climate in Dubai is characterized by low rainfall. Rain usually has a beneficial effect by washing samples in open conditions, as has been observed in numerous European tests.29 No such effect was observed in the present case; on the contrary, better performance was exhibited by the sheltered specimens. One plausible explanation for the difference is that condensation is more likely to occur in open condition surfaces, which are more rapidly cooled at nightfall, and promote corrosion. Another contributory factor is that observed in previous studies: that there was a higher proportion of sulfate in the deposits formed in sheltered conditions.2,4,16 This might act as corrosion inhibitor.
Correlation Between the Field Test Result and Guidelines for Stainless Steel Selection (EN 1993-1-4 Standard and IMOA Guide)13-14
The corrosion resistance class (CRC) system in EN 1993-1-4 standard13 and the IMOA guide14 were selected for comparison of the results because they were developed specifically for stainless steels in built environment applications. The CRC system suggests stainless steel that does not suffer structurally damaging corrosion in the particular environment. The CRC does not consider the appearance of the parts or aesthetic concerns such as rust or staining. The IMOA program is designed to help specifiers select appropriate stainless steels and surface finishes for applications where corrosion staining is aesthetically unacceptable, even if there is no structural deterioration.
The CRC system is a new approach that takes into account chloride and sulfur dioxide exposure and washing of stainless steels by rain. Exposure to marine chlorides is defined by distance from the coast. In contrast, the ISO 9223 C-class system defines the corrosiveness class by the quantitative measurement of chloride deposit rates. Washing is known from experience to be of importance for the durability of stainless steels. The CRC system was developed specifically for stainless steel in Europe because it takes washing into account. However, it is not possible for different exposure conditions to be classified into different CRC classes. If it is assumed that there is no difference between the CRC classifications for open and sheltered conditions, the test site in this study can be identified as CRC V. The field test results based on the alloying levels show good agreement with the CRC class V which recommends using UNS S34565, UNS N08926, UNS S31254, UNS S32750, UNS S32760, and UNS S32520. The results in this study also showed a good correlation to the IMOA guide that recommends using a corrosion resistant stainless steel such as UNS S32205, super duplex, super ferritic, or 6% molybdenum super austenitic. IMOA gave a wide range of possible stainless steels. A stainless steel corrosion expert with architectural experience should then evaluate the site and design, and suggest an appropriate stainless steel. Although these guidelines can be used to reduce necessary testing time, they do not describe the material behavior under practical conditions. The present field test result in a Dubai marine environment gave a lesson learnt for stainless steel selection and what to expect from an aesthetic point of view.
The effect of alloying composition on the atmospheric corrosion of stainless steel in a marine environment in Dubai was investigated by conducting a field test exposure and correlating to the alloying level in the bulk material and in the surface film of the steel before exposure.
The alloying levels in the bulk material in terms of PREN (%Cr + 3.3%Mo + 16%N) exhibited a good correlation to the extent of risk of atmospheric corrosion. For open conditions, pitting was not observed for stainless steel with high alloying levels such as UNS S31254, UNS S32750, UNS S34565, and UNS S32654, whereas pitting corrosion could occur for lower alloyed grades. For sheltered conditions, the corresponding limit was UNS S32205 or lower alloyed grades. Uniform corrosion due to merging of severe pitting occurred on the low alloyed grade such as UNS S40977 in both exposure conditions.
Three different types of aesthetic degradation were observed: discoloration, staining, and red rust. Degradation was mainly from discoloration and staining for duplex UNS S32205 or higher alloyed grades. Red rusting and staining was observed for UNS S32304 or lower alloyed grades. An increase Cr content improved the atmospheric corrosion resistance and the addition of Mo (coadded with Cr) was effective in preventing red rust and also reduced the depth of pits.
Both the bulk alloy composition and the composition of the surface film, analyzed with GDOES, showed a good correlation to the appearance rating for the shorter duration exposure of 2 y. After 4 y, the bulk composition (PREN=%Cr + 3.3%Mo + 16%N) showed the strongest correlation.
Stainless steels exposed in sheltered conditions showed a better corrosion resistance and appearance than in open conditions. Three factors are considered to contribute to this difference: a very low rainfall, more condensation leading to corrosion in the open conditions, and a higher level of beneficial sulfates in the deposits in sheltered conditions.
The atmospheric corrosion results in terms of maximum pit depth or appearance rating showed similar trends. In the severe marine environment in Dubai, it is necessary to use Mo-bearing high-Cr stainless steel for adequate atmospheric corrosion resistance. The most resistant stainless steel grades were UNS S31254 ∼ UNS S3750 < UNS S34565 < UNS S32654, which give good agreement with the CRC system in EN 1993-1-4 standard and the IMOA guide. The duplex stainless steel grade UNS S32205 may be considered for construction and architectural materials in Dubai but is likely to require more maintenance.
This work was performed within the Atmospheric Corrosion Program of Avesta Research Center, Outokumpu Stainless AB. A special thanks to DEWA for providing the test site in Dubai. Arne Bengtson and Lena Wegrelius are acknowledged for valuable discussions. Annika Almén is thanked for the evaluation of exposed samples, Arne Bengtson for GDOES analysis, and Andreas Persson for retrieving the samples.