Scratching Tool acc. to van Laar, Model 426

Description

The Scratching Tool acc. to van Laar is a ball-shaped hard metal pin with a tip diameter of 0.5 mm is mounted stably in a ballpoint pen-shaped holder made of anodised hard aluminium. The special handy design of the instrument guaranties a stable appliance during the scratching operation. The spherical point was submitted to the following endurance test: A cylinder of 100 mm dia. (4") free-cutting steel chucked in a lathe was scratched by a scratch needle at a surface speed of 20 cm/s (8") and a pressure of 50 N (11 lbs). No damage of the spherical point did occur after a run of 18000 cm (600 ft). It proved that at least 2000 scratches of 90 mm (3.1/2") length each can be made without a failure.

Purpose and application

Scratching-in of the St. Andrew's cross into test panels is a well-known preparation for short-term and endurance corrosion tests, enabling the examination of the protection of the coating against rusting of the substrate underneath the coating ("underrusting"). The Sratching Tool acc. to van Laar is a handy universal tool: for producing accurately defined incisions on the surface of the coating in preparation of forced corrosion tests, such as salt spray, continuous or intermittent immersion, accelerated weathering, gas corrosion and humidtiy tests.

Principle of the Test

For producing a specified incision in the protective coating, the test panels are scratched with the scratching needle at the beginning of the corrosion test and again at regular time intervals. These scratches are made horizontally, one above the other, the first at the bottom on the shorter side of the panel. By this method it is easy to study the progression of the "underrusting". More or less strain will have to be exerted according to the thickness and hardness of the coating in order to reach the substrate with certainty and to produce a slight deformation of the metal. For drawing a clean scratch should be about 10 cm/s (4").

Evaluation of the Test

As spreading rate the number of days is recorded it takes to form 1 mm of "underrusting". The extent of the rusting is measured from the edge of the original scratch track. If good adhesion prevails, the track width in the metal is about 0.5 mm (0.020"), with bad adhesion, however, part of the coating chips off at the edges. (The mean width of the chipped-off film can be taken as a measure for the adhesive strength of the film and for its change in the course of the test.)

In every case, it is the "underrusting" i.e. the rust below the adhering coating formed after scratching that is to be measured. The adjoining sketch demonstrates a case at which the paint coat was slightly chipped next to the scratch track.

The full extent of the spreading rust is essentially measured in both directions, the bare part is then deducted and the result pided by 2 in order to obtain the mean extent of the "underrusting".

For good adhesion is the "underrusting" index r therefore: r: r = (c - 0,5) / 2 when c = is the over-all width of the "underrusting" in mm.

Further useful suggestions as to the evaluation of corrosion tests can be found in the paper by van Laar mentioned below.

References: van Laar, J.A.W.: Die Unterrostung von lackiertem Stahl (The "underrusting" of paint-coated steel). Deutsche Farbenzeitschrift, 15 (1961), Page 56 - 67 and 104 - 117

Scope of delivery

• Scratching Tool acc. to van Laar, Model 426