Institute, SSAB and ArcelorMittal in a RFCS project. [7, 8]. Indeed, the test which gave the best correlation to field exposure on a large number of coil coated.
New cyclic test for building coil coated materials Nathalie LeBozec1, Dominique Thierry1, Per-Erik. Augustsson2, Monica Zapponi3, Dave Francis4, Mathurin Grogna5, Dorothe Koberg6, Laurence Opdam7 (1.French Corrosion Institute, 220 Rue Pierre Rivoalon, F-29200 Brest, France; 2. SSAB EMEA AB, 93 SEDG, SE-78184 Borlänge, Sweden; 3. Tenaris Siderca, Simini 250 Campana Buenos Aires, Argentina; 4. Akzo Nobel, Bressenden Place, London SW1E 5BG, UK; 5. AC&CS, CRM Group, Boulevard de Colonster B57, B-4000 Liege, Belgium; 6. BASF Coatings GmbH, ECI/CEC - A405, 48165 Muenster, Germany; 7. Tata Steel, Swinden Technology Centre, Moorgate, Rotherham, South Yorkshire S60 3AR, UK.)
Abstract: Although neutral salt spray test “NSST” (ISO 9227, ASTM-B117) is currently used to evaluate the corrosion performance of coil-coated products, it is well known that it fails in reproducing the type of degradation observed on such materials at natural weathering sites and consequently is not useful to predict the durability of the products. The main objective of the study was to define reliable testing conditions for coil-coated materials with respect to cut-edges. To do so, existing standardized tests (such as NSST, CCT-1, Prohesion test), cyclic tests used in the automotive sector and newly developed tests based on a previous work were conducted and compared to field exposures in marine and marineindustrial atmospheres. Conventional coil coated materials including a large range of substrates (e.g. HDG of different zinc thicknesses, galfan and zincalume) with different pre-treatments (Cr free, chromate) and coating systems (Polyester, Polyurethane, PVDF and Plastisol) were selected as well as less resistant products with either a thinner metallic coating, or no surface treatment or no primer. From the results, the best correlation to field was observed with a new cyclic corrosion test deriving from previous work in a simplified version. This cyclic test alternates exposure to salt solution containing chloride and sulfate at low pH with wet and dry cycles at constant temperature. Keywords: corrosion, coil coating, edge creep, accelerated corrosion tests
Coil coating products are generally tested both under accelerated and natural weathering conditions with respect to their corrosion properties at cut edges, their delamination at scribes, and with respect to their corrosion properties at open surfaces and at formed areas. Field tests are normally the most reliable but they are costly and time consuming. Accelerated corrosion tests are therefore widely used in order to: (a) develop and qualify new corrosion resistant products, (b) develop new pre-treatments and finishing processes, (c) select materials, (d) perform quality control of the final product. The accelerated corrosion test which is used, as a standard today in the building industry is the neutral salt spray test (e.g. ASTM B-117 or ISO 9227). Although this test has been used for decades to evaluate the corrosion performance of coil-coated products, it is well known that it fails in reproducing the type of degradation observed on coil-coated products at natural weathering sites and consequently is not useful to
predict the durability of the products. In the last decades, several attempts have been done in order to develop more reliable accelerated corrosion tests for the building industry [1-3]. Other accelerated corrosion tests such as the prohesion test have been developed in a more empirical way [4]. Several attempts to correlate results of existing accelerated corrosion tests (e.g. including the ABC-test and the prohesion test) with that obtained at natural weathering sites for different coil-coated materials have been performed through the European Coil Coating Association (ECCA) [5] and through a task group of American iron and steel institute [6]. More recently, a systematic study was performed at the French Corrosion Institute (FCI) in order to understand better the influence of climatic parameters on the degradation of coil coated products. From this work, a new cyclic corrosion test alternating phases with salt spray, UV and sulphur dioxide (SO2) with wet and dry transitions was proposed [7, 8]. This test was found to give a good
acceleration factor without loss of correlation to field data (e.g. 3 to 5 years of exposure in marine environment in Sweden). However, this test included phases with SO2 which levels may be difficult to control with sufficient accuracy and which implies the use of specific and costly climatic chambers. Thus, it could be beneficial to simplify the test procedure and to compare the results with existing standards in this field which was conducted in the present study. 1
Experimental
1.1 Materials Coil coated materials including a large range of substrates (e.g. hot dip galvanised steel of different zinc thicknesses, galfan and Zn55%Al) with different pre-treatments (Cr free, chromate) and coating systems (Polyester, Polyurethane, PVDF and Plastisol) were selected. In addition to conventional and commercial coil coated materials, less resistant products with either a thinner metallic coating, or no surface treatment or no primer were also tested. In
total, 26 different coil coating materials were included in the study. Sample of 100x200 mm in size were used for laboratory tests while twice larger panels were prepared for field exposure. Two scribes down to steel (e.g. one vertical of 80x0.5mm and 1 horizontal of 40x0.5mm) were applied using an Elcometer scribing tool equipped with a rectangular blade of 0.5mm. 1.2 Accelerated Corrosion tests 1.2.1 Standardized corrosion tests Table 1 presents the main characteristics of the standardized accelerated corrosion tests that are commonly used for coil coated materials e.g. the neutral salt spray test (ISO 9227), the prohesion test (ASTM G85, Annex A) and the CCT1 (ISO 11997-1). In addition, one newly developed corrosion test used by some car manufacturers in Europe has been conducted. The so called N-VDA (VDA 233-102) test has been developed in order to replace the test standard VDA 621-415, which is well-known to fail in simulating on-vehicle conditions.
Table 1: main characteristics of standardised accelerated corrosion tests Pollution Test
Salt solution
Frequenc y
Total Clmg/cm², test
T °C
NSST ISO 9227
NaCl 5 wt% pH 6.5-7.2 -1.5mL/h 80cm²
continuou s
~568
35
Prohesion test ASTM G85 AnnexA5
NaCl 0.05%wt NH4(SO4)2 0.35%wt pH 5-5.4, 1.5mL/h 80 cm²
12h
~ 2.8
24 35
CCT-1 ISO 11997-1
NaCl 5 wt% pH 6.5-7.2 1.5mL/h 80cm²
4x3= 12h/day
~284
N-VDA VDA233-102
NaCl 1%wt pH6.5-7.2, 2mL/h 80 cm²
3h/day (3 days) 9h/week
~8
1.2.2 New testing procedures One task of the project was to simplify the testing procedure developed by the French Corrosion Institute, SSAB and ArcelorMittal in a RFCS project [7, 8]. Indeed, the test which gave the best correlation to field exposure on a large number of coil coated materials included a daily phase with SO2 which implies the use of specific and costly climatic chambers. Moreover, it is well-known that the level of SO2 may be difficult to control with sufficient accuracy Thus, simplified testing procedures were proposed using an acidified solution of NaCl and Na2SO4 (pH 3). The ratio NaCl/Na2SO4 was 6 as in
35 60 50 -15 Up to 50
Approx. ToW %
Basic Duration, days
-
100
42
NC
60
42
0.8 ... ++: 0.8
