Hilti DX Corrosion Statement_20.08.2008

Corrosion behavior of DX fastener 1www.hilti.com

Corrosion behavior of DX fasteners

Schaan, August 2008

Jean-Louis Milhes Gerald Felder Michael Siemers


Objectives

This brochure is intended to give an overview about corrosion in general and an outlook on specific issues for high strength powder actuated fasteners with consequences for an adequate fastener selection.

Contend

● Definitions Page 2● Types of corrosion Page 4● Different forms of corrosion Page 5● Corrosion characteristics of DX Fasteners Page 12● DX fastener selection Page 16● Literature Page 18


Definition

Corrosion is the detrimental influence on a Material due to chemical or electrochemical Reactions with its Environment. ( DIN 50900)

Materials

This definition also includes the destruction of wood, ceramics or textiles. In practical use, the word is mostly connected with plastics and metals, therefore corrosion is associated and understood directly as "corrosion of metals".The topic "materials" also includes all factors influencing the material structure (like alloying, heat treatment, cold forming etc.) as well as those factors which influence the corrosion properties in application.

Environment

"Environment" generally means all aggregate conditions, whereas in daily use corrosion in liquid media is of most importance. Besides the concentration and composition of the liquid media (electrolyte), the most significant factors are temperature and pressure.

Reaction

Contrary to mechanical wear, corrosion is a chemical process in which metal atoms transfer from the metal state into a non-metallic character as ions in the form of soluble or nonsoluble chemical compounds. The basics of corrosion are therefore seen in physical chemistry, particularly in phase transformation reactions - thermodynamics and kinetics describe the occurring processes.

Corrosion can take place only if all three Conditions (Materials, Environment and Reaction) are present!


• chemical Reaction: e.g. oxidation

• metallurgical Reaction: e.g. hydrogen embrittlement due to hydrogen diffusion

• electro-chemical Reaction: Reaction takes place with exchange of electrons. An electron conducting media is a prerequisite (e.g. water)

Types of Corrosion


1. Homogeneous Corrosion

A... starting levelB... reduction of the thickness due to homogeneous corrosionK... grain (crystal), lots of grains determine the structure

homogeneous material reduction

Most of the commonly observed material degradation can be traced back to this corrosion form, characterised by a more or less homogeneous surface reduction. The amount of material loss due to corrosion can be approximated in laboratory scale experiments. The so-called corrosion rate is generally listed as mm/year or g/ m2 h (laboratory values).

Different forms of CorrosionDepending on the environmental conditions and material


Mean Corrosion Rate of e.g. low alloyed steel, zinc

60 - 230 µm5- 9 µmmarine

40 - 160 µm6- 10 µmindustrial

30 - 70 µm3- 5 µmtown

10 - 60 µm 1- 2 µm rural

mean surface removal/yearlow alloyed steel

mean surface removal/yearzinc coatings

(dechema handbook)Atmosphere


2. Stress Corrosion Cracking

This form of corrosion induced failure can only take place if particular media and a tensile stress are present (e.g. chlorine containing media). Existing residual stresses may be sufficient to induce stress corrosion cracking.

transcrystalline stress corrosion cracking intercrystalline stress corrosion cracking


3. Pitting Corrosion

The corrosion resistance of stainless steel materials depends on the formation of a thin and dense passivating oxide layer. Breakage of this passivating layer is possible in the presence of Halogen-ions, such as Chlorine, Iodine and Bromine. Due to the attack of these ions an increased corrosion takes place at locations where the passivating layer is interrupted. The corrosion attack takes place in the form of small pits which can propagate and destroy even thick parts in a rather short time. Therefore pitting corrosion has to be regarded as a much more severe attack and corrosion form than homogeneous corrosion. The corrosion rate increases with decreasing ph-value (acid value of the solution) and increasing oxidising ability of the electrolyte. Surface areas which show signs of mechanical treatment, or cold formed areas, accelerate the corrosion attack. Alloying with molybdenum and increasing chromium content reduces the susceptibility to pitting corrosion for chromium- and chromium-nickel steels. Pitting corrosion is not restricted to stainless steel, it may also take place on other materials creating a passive layer, e.g. aluminum.

I... passive layer

II... locally destruction of the passive layer and active corrosion afterwards in form of pits


4. Crevice Corrosion

Locally increased corrosion attack (pitting) in crevices starts because of corrosion elements which have been caused by concentration differences of the electrolyte. Another possibility is with self-passivating materials, if the oxygen is hindered in getting to the surface, and the Re-passivating is obstructed . The smaller the crevice, the more critical the situation.

I... passive layer, which can not form in the decreasing crevice

II... active dissolution starting from the location in the crevice where only a minor or no passivation occurs

III... surface contamination, deposit, washer, designed crevice, etc.


5. Contact Corrosion

A...... starting levelB1, B2.. reduction of the thickness depending on the materials resistance

Accelerated corrosion of an electrochemically “less noble” material in contact with a “noble”material. The material loss of the noble partner is reduced, the loss of surface area of the less noble partner is increased. Prerequisite for this form of corrosion is an electrically conductive connection between these two materials.

Two materials of different resistance/polarity exposed to the same media, not in direct electrical contact

Two materials of different resistance/polarity exposed to the same media, in direct electrical contact

Material 1 Material 2



Surface area ratio

Whether contact corrosion occurs depends also on the surface area ratio.

A) If the surface of the less “noble” material (1) is bigger than that of the more “noble” material (2), it will act as a very small cathode and the current density on the "large anodic" less noble material is very small. Further on, this implies also a very small corrosion rate of the „less noble“ part due to electrochemical effects.

B) But if the surface of the less “noble” material is smaller than that of the more “noble”material, corrosion rate of the „less noble“ part will be very high.




Hilti galvanized carbon steel fastener

DX fasteners are galvanized (zinc) with a zinc plating thickness of approx. 5 to 15 microns. The life time of this form of corrosion protection depends from the environmental-conditions and therefore from the corrosion rate of the zinc- layer. Most of the commonly observed material degradation can be traced back to homogeneous corrosion form, characterised by a more or less homogeneous surface reduction, as shown on page 6 .Therefore the life expectancy of galvanized fasteners and nails in wet atmospheres is very short.

High risk of failure due to Hydrogen Assisted Stress Corrosion Cracking

All HILTI DX fasteners are made out of high strength material and with that, the danger of Hydrogen Assisted Stress Corrosion Cracking is given.When zinc plated fasteners are used in wet or corrosive surroundings, the zinc plating is attacked and the fastener can corrode. Cracks will form in the fastener, which will suddenly failed with static load, even if very small loads are applied. This phenomenon, leading to high risk for the construction is unpredictable and not controllable.

Corrosion characteristics of DX Fasteners


Hydrogen Assisted Stress Corrosion Cracking

●Brittleness due to hydrogen dissolved metal lattice

●Inter-crystalline (between the grains)

●Tensile stress

●Associated with hardened high-strength steel

Incorrect Application (Example)

Zinc-plated powder-actuated fastener used in corrosive industrial environment

Stress Cracks


Hilti X-CR / X-BT stainless steel fastener (CR500 material)

Tests carried out at independent authorities (FMPA Stuttgart ([5], RWTH Aachen [6]) clearly indicated the superior properties of the CR500-material if compared to AISI 316 (A4) or AISI 304 (A2). The superior properties with regards to pitting potential are mainly due to the higher amount of molybdenum, nickel, chromium and the content of nitrogen in the steel. CR500-material can consequently classified in the same corrosion category than AISI 316 (A4).

As results, available from field tests in e.g. industrial atmosphere, tunnels and in see water since many years, the adequate resistance of AISI 316 (a4) steel grade for use in "corrosive conditions in industrial and marine air" has been concluded. This resistance exists for the CR500 material with great certainty.


Contact corrosion

Contact corrosion is, where stainless steels are concerned, no matter of discussion. Stainless steels are higher in the galvanic series, i.e. nobler, than most generally used materials such as aluminum, zinc and steel. Thus stainless steel in contact with these materials obtains cathodicprotection. The contact therefore generally has a favourable effect on the corrosion properties of the stainless steels. The “noble” stainless steel fastener implies a very small corrosion rate of the „less noble“base and fastened material due to electrochemical effects as described on page 11.

Heavy corrosion environment

In some heavy corrosive environment, e.g. road tunnel [3] in the alps (salt and air pollution), applications in particularly chemical industry, where chlorides and acid compounds are combined and the fastening is high safety relevant, the usage of X-CR material is not permitted. This combination of "maritime" as well as more acid and oxidizing working constituents in the electrolyte film are the reason, why conventional stainless steels of the class AISI 304 and AISI 316 can suffer pitting corrosion [3] and in further consequence stress corrosion cracking. This is one of the most dangerous form of corrosion. This corrosion induced failure can only take place if particular media and a tensile stress is present. Existing residual stress may be sufficient to induce stress corrosion cracking.


DX fastener selectionThe subject of corrosion has a major influence on the suitability of a fastener and therefore also on fastener selection.For non-safety relevant applications, zinc plated fasteners made of normal carbon steel can be used without restriction.For safety relevant, permanent fastenings following table shows the recommendations for use ( ) under different atmospheric conditions.

Outside, coastal area or industrial atmosphere without chlorides

possibleInside rooms without condensation and corrosive gases

possibleShort term exposure to weather (i.e. during construction)

Highly corrosive surroundings (indoor swimming pools, highway tunnels)

Fastener

Inside with heavy condensation

X-CR / X-BT stainless steelZinc plated carbon steelCondition for use


Fasteners that are used in wet areas must be at least as noble or better, nobler than the fastened part. The effect of contact corrosion is shown in the table below.

Negligible or no corrosion of fastenerHeavy corrosion of fastener

Construction steel (uncoated)

Stainless steel sheet

Fastener

Aluminum alloy

Galvanized steel sheet

X-CR / X-BT stainless steelZinc plated carbon steelFastened part / Base material

General statement for offshore applications:Use HILTI galvanized DX- fasteners and nails only for dry indoor applications. In highercorrosive and/ or wet atmospheres the usage of HILTI X-CR fasteners is recommended.


1. Corrosion behavior of DX-fasteners; G. Felder; September 2004

2. Corrosion behavior of stainless steel DX fasteners in carbon steel; G. Felder and M. Siemers, Schaan, September 2005

3. Status report 2005: The use of corrosion-resistant fasteners for direct fastening applications in seawater; Report TWU-32/06; B. Borufka, G. Felder, January 2006

4. Piping and electrical Installation systems, a short introduction to corrosion; G. Felder, M. Boretius, P. Jokiel; October 1996

5. Corrosion behavior of X-CR Fasteners, Report No. VI.10.1.7c; FMPA Stuttgart; July 2000

6. Resistance of bolts and grid fasteners made of stainless steel to a seawater application; RWTH Aachen; January 2005

Literature

Documents

Hilti DX Corrosion Statement_20.08.2008