Weathering testing and material durability

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Weathering testing and material durability

Weathering testing and material durability

The behaviour of sheet metal materials and structures in different atmospheric conditions and estimating their long-term durability are two critical factors when assessing a lifespan for products made of sheet metal. Typically, the durability of sheet metal products is estimated based on short-term accelerated weathering tests.

At SMC, we conduct weathering testing for materials and offer expert services on issues related to material technology and long-term durability of different materials, mostly sheet metal products. Our weathering test laboratory is equipped with three corrosion testing cabinets with which materials can be tested with continuous salt spray as well as with cyclic corrosion tests.

For evaluating the effects of temperature changes and relative humidity on materials, the SMC climate chamber is the appropriate equipment:  temperature changes between -80 °C and +100 °C can be programmed, and also the relative humidity within the chamber is adjustable. The effects of sunlight UV radiation  and humidity on organic materials, such as paints and plastics, can be simulated in accelerated weathering tests.

Corrosion tests

Accelerated corrosion tests

When corrosion resistance and corrosion proneness of different materials are studied, salt spray tests are commonly used. A salt spray test can be continuous or cyclic. In cyclic corrosion tests,  test samples are subjected to certain conditions in repeated cycles; these cycles can consist of e.g.  alternating salt spray and dry conditions.

At SMC, we have three corrosion testing chambers: the Harshaw –chamber can be used for continuous salt spray testing, and the Q-FOG –cabinets are suited for both continuous and cyclic salt spray tests. In addition, the cabinets can be used various condensation and humidity tests.

Continuous salt spray (e.g. ASTM B117)

In salt spray tests, samples are sprayed continuously with water solution containing 5 % sodium chloride. The temperature within the cabinet remains at 35 °C throughout the test. The test time depends wholly on the tested material, as for some materials only a few hours testing time is adequate and for some a few months is needed.

Usually, after the test has been finished, the samples are cleaned of all salts accumulated on the surfaces. Washed samples are then photographed and, when needed, the amount of the loosened coating is determined. How the samples are evaluated is based on each specific test requirement. For example, normally, in the case of paint-coated samples, the following items are checked: the amount of white and red rust from cuts made on the coating, the quality of the remaining coating and the amount of the coating loosened. Furthermore, changes in surface quality, e.g. blistering, are recorded.

Prohesion (ASTM G 85 94)

Prohesion is a cyclic test where test samples are alternately sprayed for an hour and dried for an hour. During the spraying phase, a solution containing ammonium sulphate (0,35 %w) and sodium chloride (0,05 %w) is sprayed into the chamber at temperature of  25°C. During the drying phase, air of 35°C is blown into the chamber. The test time depends on the tested material and can be altered according to the test specifications. After testing, the samples are washed and evaluated in a similar manner as the salt spray test samples.

Q-FOG CCT 600 and CCT 1100

Q-FOG is a corrosion testing cabinet which can be programmed to work in a cyclic mode. With Q-FOG, all the relevant corrosion tests can be performed. In addition to continuous salt spraying and prohesion, Q-FOG is suited also for different multi-phased cycles containing moisture or condensation combined with salt spray and drying. At SMDC, we have two Q-FOG devices: CCT 600 and CCT 1100.

Maximum temperatures for different functions:

  • spraying, max 60°C
  • drying, max 60°C
  • 100% humidity, 35°C – 60°C

Capacity of chamber:

 CCT 600CcT 1100
total capacity640 l1103 l
cap. without lid511 l857 l

Capacity for planar samples:

 CCT 600CCT 1100
100 x 300 mm128 pcsc. 200 pcs
75 x 150 mm160 pcsc. 260 pca

Suitable for most ISO, ASTM, BS, DIN and automotive etc. industry standards like: ASTM G 85 94, ASTM B117, ISO 7253, and ISO 11997-1.

Harshaw salt fog cabinet

With Harshaw salt fog cabinet continuous salt spray tests can be performed (not suitable for cyclic tests).

Capacity of corrosion cabinets

ChamberCCT 600CCT 1000Harshaw
Capacity without lid511 l857 l849 l
Planar samples 10 x 15 cm136 pcs200 pcs120 pcs
Max sample size, 2 pcs (lenght-width-height)71 x 45 x 45 cm82 x 62 x 40 cm68 x 48 x 49 cm
Max sample size, 2 pcs (l-w-h)104 x 20 x 45 cm140 x 25 x 40 cm107 x 25 x 49 cm

Weather conditions causing damage on materials

How weather conditions damage materials?

In accelerated weathering tests the harmful effects of weather conditions are simulated, and they are utilized to predict the durability of materials used outdoors. Rain and humidity can be simulated with a condensation system and/or water spray. The damaging effects of sunlight are simulated by UV-fluorescent lamps.

Simulating moisture and rain

In outdoor conditions, materials are regularly subjected to moisture. Most of the moisture is caused by condensed humidity, not actual rain. The simulation of these conditions can be achieved by a condensation cycle: water is vaporized from the water tank positioned on the bottom of the chamber. This vapour will keep the chamber’s relative humidity level close to 100 %. As the back side of the test samples is in contact with the cooler room air, the water vapour will condense as drops on the face of the samples. The drops are condensed water, very clean and basically distilled water – this will improve the  repeatability of the test and also prevent samples from getting dirty.

Simulating sunlight

In the QUV-device, the effects caused by sunlight are simulated by fluorescent UV-lamps. These lamps are normally divided into UV-A and UV-B lamps, depending on the wavelength of the radiation they emit. Even though UV-light covers only approx. 5 % of sunlight it is nevertheless causing most of the photochemical degradation of organic materials (e.g. paints, varnishes, plastics and rubbers). The wavelength range of UVA -lamps is c. 295 – 365 nm, and the correlation with the UV-radiation of the sun is good. The radiation of  UVB -lamps is shorter in wavelength and thus more efficient, but it does not fully correlate with the spectrum of sunlight.

UV testing

In the QUV-chamber, accelerated weathering tests can be performed where the effects of sunlight and moisture are simulated with test samples. With this device it is possible to quickly achieve the harmful effects which would take months or even years to achieve in the actual outdoor conditions. The damages detected in the test samples can be e.g. fading, chalking, chipping, clouding, blistering, de-glossing, frailing and embrittlement. The total test time depends on the material and standards, but usually for UVA-lamps it is longer than for UVB-lamps. Cycle lengths and temperatures can be adjusted according to standards.

QUV is a cyclic adjustable UV-chamber with following functions:

UV-light in temperature range 50°C – 80°C

  • wavelength range with A-lamps, c. 315 – 365 nm
  • wavelength range with B-lamps, c. 280 – 315 nm

Intensity of the radiation is adjustable, it is dependent on the test and laboratory temperature. Normally:

  • A-lamps c. 0,7 – 1,3 W/m2 / 340 nm
  • B-lamps c. 0,5 – 1,4 W/m2 / 313 nm

100 % humidity in temperature range 40°C – 60°C
water: drop nozzles, spray: ionized water

Capacity for planar test samples: 75 x 150 mm, 50 pcs

Suitable for most ASTM, ISO, DIN, JIS, SAE, BS, ANSI, GM etc. standards.

Examples
Typical cycles for testing e.g. coil coated sheet metals:

A-lamps, standard  ISO 4892-3, Plastics.
Methods of exposure to laboratory light sources. Part 3: Fluorescent UV lamps
8h UV-phase; 0,72 W/m2 / 340 nm , temperature 60°C
4h condensation phase; temperature 50°C
typical test time 5000 – 10 000 h

A-lamps, 0,89 W/m2 / 340 nm,  standard EN 13523-10 , coil coated metals.
Test methods. Part 10: Resistance to fluo​rescent UV radiation and water condensation.
4h UV-phase; 340 nm , temperature 60°C
4h condensation phase; temperature 40°C
typical test time 2000 h

B-lamps, standard  ISO 4892-3, Plastics.
Methods of exposure to laboratory light sources. Part 3: Fluorescent UV lamps
8h UV-phase; 0,59 W/m2 / 313 nm , temperature 70°C
4h condensation phase; temperature 50°C
typical test time 2000 h

Condensation testing

The condensation chamber is typically used to test coil-coated sheet metals. Strips of sheet metal are subjected to either continuous or cyclical condense stress, where water is condensed on the face of the test samples. Damages visible on coatings can be e.g. blistering, colouring, softening, wrinkling and embrittlement. Typical test time is 1000-1500 h, intermediate check-ups performed according to ISO 4628/2, e.g. every 250 h.

QCT is a cyclically adjustable condensation chamber, with highest possible condensation temperature at  60°C and drying temperature of 100°C.

Capacity for planar test samples:

  • max size 1200 x 150 mm, capacity 4 pcs
  • 75 x 150 mm, capacity 64 pcs
  • 100 x 150 mm, capacity 48 pcs

Suitable for most ISO, ASTM, BS and automotive etc. industry standards.

Examples

SFS-EN ISO 6270-1 Paints and varnishes. Determination of resistance to humidity. Part 1: Continuous condensation
Temperature 38°C
Temperatures of 49 °C and 60 °C are recommended if 38 °C is too low.
Humidity 100%

ASTM D2247 Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity
Temperature 38°C
Humidity 100%

Humidity and temperature changes

How changes in humidity and temperature affect materials?

In nature, materials and structures are subjected to a wide range of temperatures and weather conditions. The behaviou​r of various materials in rapidly changing conditions can be studied with the climatic test chamber of SMC. The changes in the temperature and the humidity of the climatic chamber can be cycled according to test specifications, temperature range is between  -80 +100 °C and the relative humidity of air between 10 – 95 %.

Outdoor testing

Sheet Metal Centre has two outdoor testing areas where the durability of test samples can be studied. One of the test areas is located in a forest, and it is used to study how materials endure conditions of Finnish forests, e.g. effects caused by various plant growths, pollens, solar radiation and moisture. The other test area is situated in the yard outside SMC, which enables accelerated outdoor tests.

SCAM-test (simulated corrosive atmospheric breakdown)

In a Scab-test (ISO 11474 Corrosion of metals and alloys, Corrosion tests in artificial atmosphere, Accelerated outdoor test by intermittent spraying of a salt solution (Scab test)) the samples are subjected to actual outdoor conditions. Typical testing angle for samples is 45 degrees. The samples are sprayed twice a week with 30 g/l NaCl-solution. If air temperature is  below -10°C, salt spraying is not done.

In the Scab-test, a zinc sheet is used a reference for corrosion, but it is possible also to use other references provided by the client. As outdoor conditions may vary greatly, temperature and weather condition data on all spraying days is recorded in a test diary.

Colour and gloss measurements

Colour and gloss measurements are used widely in quality control. They can also be utilized to follow changes in material colour and gloss when the material is ageing. Changes are measured especially in combination with accelerated weathering tests and outdoor tests.

Visual checking is important when testing colour changes, but utilization of testing devices will provide numerical values which do not vary from person to person. At SMC, the tester geometry is diffuse/8° (D/8). Colour scale, light source and observation angle can be chosen according to test specifications.

Colour and gloss measurements can be used to measure a wide range of materials and products: metals, plastic, cloths, wood, paint, pigments – imagination is the only limiting factor. The most reliable result is achieved when testing on a planar sample surface, but since the tested area is small when using a tester device, it is possible to test products in different shapes. The gloss tester at SMC is portable, which allows measurements anywhere. The colou​r testing at SMC must be performed on site.

Spectral photometer Datacolour 600 TM

  • measures product colour
  • suitable for most materials and products
  • testing geometry Diffuse/8° (D/8)
  • colou​r scale, light source and observation angle adjustable
  • testing without gloss component and with gloss component is possible (SPIN/SPEX)
  • measurement orifices in use are 6,6 mm, 9 mm and 30 mm

Gloss tester micro-TRI-gloss

  • measures product gloss
  • suitable for most materials and products
  • testing geometries 20°, 60° and 85°
  • portable, testing can be done anywhere

HÄME UNIVERSITY OF APPLIED SCIENCES