Corrosion testing The correct introduction

of scribe marks

Ingrid Bloss Zehntner GmbH Testing Instruments

Switzerland

Definition

Corrosion is a chemical or electrochemical reaction between a material, usually a metal, and environmental factors that produces a deterioration of the material and its properties. Metals have a natural tendency to return to their lowest energy states and therefore combine with oxygen and water, both of which are present in most natural environments, to form hydrated iron oxides (rust), similar in chemical composition to the original iron ore. Affected metals:

Iron,

Aluminum, Copper, Magnesium, Titanium

Definition

Affecting factors • Moisture (Humidity) • Temperature • Oxygen • Air Pollution/Contamination • Water Quality • Lean MWF (metalworking fluid) dilutions • Acids/Bases/Salts • Concentration differences across metal surface • Shape of the object (crevices) • Contact between dissimilar metals

Motivation

reduction of metal thickness

loss of mechanical

strength

structural failure/ breakdown

hazards or injuries to

people (e.g. bridges, cars,

aircraft).

contamination of fluids in vessels and pipes

perforation of vessels and pipes, escape of

their contents

possible harm to the surroundings

mechanical damage to valves, pumps, by solid

corrosion products

Detrimental effects of corrosion

reduced value of goods due to deterioration

of appearance

Change/loss of important

surface properties (frictional properties,

electrical conductivity, surface reflectivity etc.)

Motivation

Economic consequences Increased costs due to • Replacement • Overdesign • Maintenance

Social consequences • Safety hazards • Health risks • Depletion of limited

resources

Corrosion should be preventend whenever possible.

Motivation

It is crucial to conduct reproducible tests regarding the protective properties of innovative coatings in combination with the intended substrates. Since the majority of coatings nowadays are very hardwearing, it is mostly useless to test the unblemished surface. Introducing defined scribe marks is an important factor in achieving reproducible results

Standards for corrosion testing and the introduction of scribe marks

Standard Scribing tool Scribe layout

DIN EN ISO 4628-8 not defined not defined

DIN EN ISO 9227 Scribing tool with hard tip, and square or trapezoid profile

vertical distance to sample edge 0.2 – 1 mm

ISO 20340 milling cutter, 2 mm horizontal distance to sample edges defined length 50 mm

DIN EN ISO 17872 (2007) square profile U- or V-shaped

x-form, t-form or single line length 50 – 70 mm width min. 0.2 mm

TL/TP-KOR-Stahlbauten (2002) not defined parallel to lengthwise edge of sample distance to sample edges defined

VDA-Prüfblatt 621-415 (1982) scribing tools according to van Laar or Clemen, single blade cutter, no square profile

vertical

Iconsistent procedures

Evaluation methods

The evaluation methods for the corrosion tests described in the individual standards are also not uniform. In some standards only the corrosions directly in the scribe area is evaluated, whereas in others also the amount of delamination is taken into account. Some standards demand average values, some maximum values.

Inconsistent evaluation

.

Objective comparison of individual methods

impossible

Standards

It is therefore difficult to achieve comparable results in corrosion testing. Two recommended publications dealing with scribe marks and their effects on the test results . Both are not covering the subject entirely, but are nonetheless giving a good overview and helpful information on scribe marks.

ISO 17872

Guidelines for the introduction of scribe marks through coatings on metallic panels for corrosion testing

Describes methods of scribing coated samples for corrosion tests, for coating thicknesses of less than 500 µm

Intended as a guideline only,

Based on the results of a collaborative trial

Only studies the effectiveness of the respective scribing tools

No subsequent corrosion tests.

ISO 17872 Cross section of scribe marks

V-Shaped U-shaped

ISO 17872 Surface shape of scribe marks

x-shape t-shape single line

ISO 1787 Scribing tools

Cutter 301 Clemen Van Laar Sikkens Mechanical Tool S22B

V U U V ?

width 1 mm

radius 0.25 mm

width 0.5 mm

Procedure

The procedure is described in detail. Most important factor in the introduction of scribe marks:

Mark must penetratesthe coating system right through to the substrate.

Scribing tool trial

The annex of the standard describes the results of a trial where some (but not all) of the described tools have been tested for the uniformity of their results and their ability to penetrate the coating sufficiently. The only tool which is able to penetrate all tested dry film thicknesses is the mechanical milling tool S22B, all other tools need 3-5 actions until the coating is penetrated entirely.

Comparative Study IKS

Rudolf et. al German Institute for Corrosion Protection IKS, Dresden Germany, 2008 • best comparison of the different scribing methods

• focusses mainly on the actual corrosion testing

• compares the influence of different methods of artificially inflicting damages on coatings in dependence of the scribe layout (surface shape) and scribe depth as well as surface preparation

Sample preparation and corrosion tests

Several coating systems tested on • steel • galvanized steel (different galvanization methods) • aluminium The samples have been subjected to the following corrosion

procedures: • alternating cycles salt spray – condensing waters (VDA 621-415) on

steel and galvanized steel • neutral salt spray test (DIN EN ISO 9227-NSS) on galvanized steel • acetic acid salt spray test (DIN EN ISO 9227-AASS) on aluminium • filiform corrosion resistance test (DIN EN 3665) on aluminium

Scribing tools and cross sections of their respective scribes

Commercial cutter (Cutter 301)

result

Scratching tool according to Clemen blade width 1,0 mm

result

Scratching tool according to van Laar tip radius 0.25 mm

result

Scratching tool accroding to Sikkens blade width 0.5 mm

result

Adhesive tape, 0.5 mm width

result

Milling tool with circular saw blade

result

Milling tool with staggered tooth blade

result

Overview of all results

As already anticipated by looking at the cross cuts of the scribe marks the results after exposure to corrosion procedures are very diverse.

Conclusion / Recommendations

• Depending on the coating system the shape and design of the scribing tool can have a considerable impact on the result of the corrosion test and it is not easy to find the best method for one's individual requirements.

• Since there are so many established scribing methods in use and the existing ISO 17872 standard is not covering the subject comprehensively there is too much room for variation.

Conclusion / Recommendations

• All involved parties should agree upon the most suitable scribing method for the coating system under inspection before conducting elaborate and time consuming corrosion tests.

• The use of a universal tool with interchangeable cutters offers an economic solution to introduce a variety of scribe marks and gives the users the possibility to choose the optimal scribing method for their coating systems.

• It should furthermore be considered to revise the standards dealing with corrosion testing and scribe marks with the aim to give uniform descriptions of tools, procedures and evaluation. Optimal comparability can only be achieved if these standards are on the same level.