Under-film corrosion: Filiform and scab corrosion

A professionally painted metal surface should never be corroded; the function of the paint is actually to protect the metal support from corrosion other than being an aesthetic factor. Nevertheless, painting cycles are sometimes performed on supports that have not been treated properly.

In this case, the symbiosis between paint and support becomes extremely vulnerable and a small grain of dust on a scarcely painted edge or any other cause is enough to start the corrosion process. For example, if a car is hit by a stone, its paint cracks: corrosion propagates under the paint layer with short-term destroying effects, on the sheet metal first then on the paint itself. The real enemy that traditional salt spray chambers fail to detect is this kind of corrosion that propagates under the paint film and whose effects may be either a scab corrosion or a filiform corrosion.

Dry corrosion test

The ISO 7253 standard of 1984 at international level (ISO means International Standard Organisation) sets forth both the application modes of salt spray tests and their limits. This standard actually states that salt spray tests are not valid as corrosion tests (not involving a real exposure to environmental agents) but they are only a sort of behavioural test to check the quality of different lots of painted products as well as the quality of the painting processes.

From what has been said so far, it clearly emerges the need of laboratory tests aimed at detecting and highlighting the two forms of under-film corrosion (filiform and scab) in a fast, easily reproducible manner. This test already exists: it is the so-called Dry Corrosion Test.

The procedure is the same as that followed for salt spray tests, but the two salts used are different: sodium chloride and ammonium sulphate. Moreover, in dry corrosion tests, any wetting cycle is followed by a drying cycle with de-humidified hot air. A complete test cycle may last from 500 to 1000 hours depending on its severity.

Dry Corrosion Test Cabinet©

  • Plastic execution
    DCTC systems are completely made of plastic, anti-corrosive materials. The test compartment has been moulded from a single plastic sheet to ensure long-life and perfect seal. External panels are completely removable for easy maintenance.
  • Heating
    Shell-type heating system insulated with self-extinguishing glass wool.
  • Access hood
    • completely transparent PVC hood
    • reversed V-form to allow a better inspection of the test environment and a better flowing of condensation drops to prevent them from falling onto the specimens under test
    • servo-assisted opening/closing by means of small gas pistons to make the hood lighter
    • full-tight silicon gasket for perfect seal between the access hood and the test compartment
  • Air humidification
    • by blowing compressed air into a thermo-regulated bath
    • humidifier made of AISI 304 stainless steel
    • indicator of relative humidity (P models)
  • Saline solution tank
    • made of anti-corrosive PVC
    • capacity: 120 litres for 7-day tests (according to most standards)
    • possible connection to an external tank (optional)
  • Automatic tests
    All tests are performed either with continuous or alternate salt spray; they may be also conducted automatically. In the first case, the humidification of compressed air is commanded automatically; in the second case, it is automatically switched off (dry air).
  • Saline solution atomisation
    The saline solution is sprayed inside the test chamber through an adjustable universal nozzle made of leucite (an anti-corrosive material), mounted on a plexiglass support. It is possible to adjust the intensity and precision of the spray by adjusting the speed of the membrane pump (atomization). At the end of tests, it is also possible to reduce the fumes inside the chamber by means of compressed air (P models) so as to discharge all residual fumes before opening the hood.
  • Control and regulation system (P models)
    Based on a µPLC structure, it has a control panel consisting of a display with 5-function keyboard allowing the setting of the necessary parameters to perform automatic and cyclical tests. The following parameters may be set and displayed on the control panel:
    • temperature
    • number of cycles
    • test duration
    • membrane pump
    Output data are saved by the µPLC and displayed on the monitor. Alert and warning functions are also available to ensure a correct chamber operation. They indicate:
    • demineralises water lack
    • compressed air lack
    • saline solution lack
  • Control and regulation system (basic models)
    If consists of digital timers and controllers for dry corrosion - continuous or alternate spray - tests.

Standard functions

  • Dry corrosion test
    This kind of test consists of salt spray cycles followed by low relative humidity drying phases; it is performed according to UNI 9399 and UNI 9590 norms to highlight in a short time, and above all in a reproducible way, all under-film corrosion phenomena (filiform and scab). Particular care has been given to the generation of the salt spray which deposits onto the specimen in the form of uniform small drops, the very beginning of a powerful corrosive attack when water is evaporated during the drying phase.
  • Prohesion test
    This test consists of repeated cycles of saline solution sprays at 25°C for 1 hour, followed by a drying phase at 35°C lasting 1 hour.
  • Continuous salt spray (conventional salt spray)
    It includes all the ASTM, MIL STD standard, etc.
  • Alternate salt spray
    It includes DIN 50.021 and DEF standards, whose cyclical variations may be divided into two phases:
    • salt spay humid phase
    • drying phase without air flow
  • Cyclical tests
    P model chambers equipped with µPLC programmer allow the setting and performance of temperature cycles.
  • Control via PC
    A dedicated software allows test management and control (local and remote) via PC. The microprocessor is interfaced through RS232 serial port. The system allows the following functions:
    • Windows 98/2000 or NT environment
    • numerical and graphic control of input and output data
    • modification of input data during test execution
    • personalised selection of scales and curves for the charts to be displayed and printed
    • colour prints
    • test data storage for subsequent export and processing by other environments
  • Graphic recorder
    6 channels micro-processor-based for temperature and relative humidity.
  • PT100 probes
    Additional for graphic recorder
  • Specimen supports
    • 19-position racks for panel-like specimens, with gradual inclination grooves
    • Support platform placed at the bottom of the test compartment for large-size specimens.
    • Set of no.4 additional hanging supports for specimens:
      • No. 1 set for DCTC600 (P)
      • No. 2 sets (max.) for DCTC1200 (P)
  • Single column demineralisation system
    It is used de decacify water thus allowing running water to be used to supply the humidifier. Spare columns are available.
  • Air compressor
    Complete with tank to be used when no other source of compressed air is available.
  • Additional tank
    Made of PVC, capacity 200 litres, to be placed outside the chamber. Sizes: 600 x 800 x 700mm (WxDxH).
  • Digital pH measuring system
    Complete with single-tube electrode.
  • Wheels mounting
    Complete with adjustable supports for chamber levelling and for chamber drainage (when definitely positioned).
  • Wetting test
    For P models only

For more information on Dry Corrosion Test Cabinets© contact Trevor Leake.