CORROSION HANDBOOK 06/2021
FOREWORD Corrosion is a ubiquitous natural process. Most of us, at some point in our everyday lives, become familiar with the effect that corrosion has on rusted steel parts. Corrosion has a huge economic impact. About a fifth of the world’s annual steel production simply goes towards replacing parts damaged by corrosion. Even though it may involve higher up-front cost, correct and efficient corrosion protection at source helps save money and resources in the long run.
CONTENTS 4 4 6 6 7 8 9 11 12 1 1.1 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6 13 13 14 17 2 Hilti corrosion performance assessment and product qualification methods 2.1 Purpose of corrosion testing 2.2 Lab facilities/tests 2.3 Outdoor field tests 19 20 22 26 3 3.1 3.2 3.3 28 4 How to assess corrosion in a specific environment and application 4.1 Factors influencing atmospheric corrosion 4.2 Assessment of corrosivity for zinc and ZM coated products 4.
1 BASICS OF CORROSION 1.1 What is corrosion? Corrosion is the physicochemical interaction between a metal and its environment, which results in changes in the metal’s properties and may lead to significant functional impairment of the metal, the environment, or the technical system of which they form a part (see ISO 8044:2010). We only talk about corrosion when there is a change in the metal’s or system’s properties that may lead to an undesirable outcome.
These two partial reactions can take place on the metal surface in a fairly even distribution leading to uniform attack (see section 1.2.1). Alternatively, they can occur locally and separately, leading to localized forms of corrosion such as pitting corrosion.
1.2 Forms of corrosion 1.2.1 Uniform corrosion / shallow pitting corrosion Uniform corrosion removes a metal’s surface almost evenly. The partial reactions (metal dissolution and oxygen reduction) are statistically distributed over the surface, leading to a largely homogenous dissolution of the metal and uniform formation of corrosion products (e.g. red rust on steel). The extent of this form of corrosion can usually be well estimated on the basis of previous experience.
1.2.2 Pitting corrosion Pitting corrosion is a localized form of corrosion that leads to the creation of small holes or “pits” in the metal (see Fig. 5). This form of corrosion is mainly found on passive metals. Passive metals and alloys, such as aluminum, titanium and stainless steel owe their corrosion resistance to a thin oxide layer of only a few nanometers on their surface. The corrosion process starts with a local breakdown of the passive layer.
1.2.3 Crevice corrosion Crevice corrosion refers to corrosion occurring in cracks or crevices formed between two surfaces (made from the same metal, different metals or even a metal and a non-metal). This type of corrosion is initiated by the restricted entrance of oxygen from the air. The access of oxygen into the crevice area is hindered, whereby the cathodic partial reaction (see 1.1) preferably takes place only outside the crevice.
1.2.4 Environmentally induced cracking Stress corrosion cracking (SCC) Stress corrosion cracking (SCC) is a combined mechanical and electrochemical corrosion process that results in cracking of certain materials. It can lead to unexpected sudden brittle failure of normally ductile metals subjected to stress levels well below their yield strength. The stress in a material can be either applied (external) or residual (internal) and sufficient to initiate an attack of stress corrosion cracking.
Hydrogen-assisted cracking Hydrogen-assisted cracking is caused by the diffusion of hydrogen atoms into the metal. The presence of hydrogen in the lattice weakens the mechanical integrity of the metal and leads to crack growth and brittle fracture at stress levels below the yield strength. Like stress corrosion cracking, it can lead to sudden failure of metal parts without any detectable warning signs. In common applications, hydrogen damage is usually only relevant for highstrength hardened steels.
1.2.5 Intercrystalline (intergranular) corrosion Intercrystalline corrosion is a special form of localized corrosion, where the corrosive attack takes place in a relatively narrow path, preferentially along the grain boundaries in the metal structure. The most common effect of this form of corrosion is a rapid mechanical disintegration of the material (loss of ductility). Usually it can be prevented by using the right material and production process.
1.2.6 Galvanic (contact) corrosion Galvanic corrosion refers to damage caused by two dissimilar metals having an electrically conducting connection while being in contact with a common corrosive electrolyte (e.g.: humidity in the air). In the electrochemical model of corrosion, one of the two partial reactions (anodic metal dissolution and cathodic oxygen reduction) takes place almost exclusively on one metal.
2 HILTI CORROSION PERFORMANCE ASSESSMENT AND PRODUCT QUALIFICATION METHODS Hilti conducts comprehensive laboratory and field corrosion tests to assess the corrosion protection of its products. Thanks to in-house research Hilti has a wide variety of tested corrosion protection solutions for different environmental conditions. Many methods for testing corrosion resistance are specific to particular materials and are based on conditions prevailing in certain environments.
2.2 Lab facilities/tests At our in-house research facilities, Hilti performs the most relevant lab corrosion tests available for our products (see Fig. 13). Fig. 13: Picture from our corrosion lab with various corrosion testing chambers. QJ V FR DWL =0 7LPH XQWLO UHG UXVW KRXUV Neutral salt spray test: EN ISO 9227, ASTM B117 The salt spray test is one of the oldest and most widely used accelerated corrosion tests.
Cyclic corrosion test: EN ISO 16701 In the cyclic corrosion test EN ISO 16701, temperature and relative humidity are varied to simulate typical wet/dry cycles like those taking place in natural outdoor environments. Additionally, the samples are sprinkled with a dilute sodium chloride solution (1%) twice a week to induce corrosion.
Humidity test: EN ISO 6270/ ASTM D2247, and with sulfur dioxide (EN ISO 6988, ASTM G87) In the humidity test, samples are exposed to an atmosphere with 100 % relative humidity. This test can be combined with the addition of a certain amount of sulfur dioxide gas. This creates a highly corrosive and acidic environment that simulates the effect of heavy industrial pollution (see Fig. 20).
2.3 Outdoor field tests Corrosion protection of products can be assessed most accurately by exposure tests of specimens and products in real atmospheric environmental conditions. For this purpose, Hilti conducts multiple ongoing outdoor field tests for its products at various sites all over the world, in conditions ranging from cold temperate to tropical, and from coastal to industrial and even offshore atmospheres (see Fig. 24).
From the beginning of the 1980s until 2005 Hilti also carried out extensive studies on the corrosion behavior of various materials in road tunnels in the Alpine region. Our long-term observations have given us the ability to improve the performance of our products in these highly corrosive surroundings and thus supply our customers with more effective fastening systems for use in these environmental conditions. The high-alloyed stainless steel grade 1.
3 HILTI CORROSION PROTECTION SOLUTIONS The corrosion protection solutions applied to Hilti products, their typical corrosion behavior and their suitability for certain applications are described in this section.
3.1 Corrosion and corrosion protection of carbon steel The unalloyed steel (i.e. mild steel or carbon steel) from which the majority of our fastening and installation products are manufactured requires corrosion protection. In most environments, the corrosion rate of carbon steel (typically around 20 µm/a in a rural outdoor atmosphere and rising to more than 100 µm/a in coastal environments) is usually too high for a satisfactory outdoor application.
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Sherardizing / thermal diffusion Process description Sherardizing is a method of zinc coating utilizing a thermal diffusion process. The steel parts are placed in a drum containing Zn powder and then heated to temperatures above 320 °C. The Zinc is not liquid, and the coating forms by thermal diffusion of the Zn powder into the steel parts. Coating thickness The achievable coating thickness ranges up to 45 µm.
Another severe form of corrosion relevant for stainless steel is stress corrosion cracking. Austenitic stainless steel can be prone to this form of corrosion under specific highly aggressive environments such as in indoor swimming pools. In such cases, highly corrosion-resistant grades of stainless steel must be used for some applications, e.g. grades with a molybdenum content of more than 6 %. You will find more information about selecting stainless steel grades in section 4.
MATERIAL NUMBER Description The system of numbering materials in accordance with EN 10088-1:2014 is used in several countries. Each number has five digits, such as 1. 4404 The first digit 1 means steel, the second and third digits 44 mean chemically resistant steels with Mo, and without Nb or Ti.
TERMS V2A AND V4A – DESIGNATION ACCORDING TO EN ISO 3506-1:2009 Description The terms V2A and V4A date back more than 100 years and have their origin in the designation of the first trial productions of stainless steel. The V stands for “Versuch”, which is German for “test or trial”, and the A for “austenite”. V2A denotes a Cr/Ni alloy and V4A a CrNiMo alloy. The terms are still used as synonyms for stainless steel in some countries.
3.3 Prevention of galvanic corrosion Galvanic corrosion (described in section 1.2.6) can be avoided by the right choice of material combinations. This, however, is not always possible and sometimes other measures have to be considered. One example is galvanic separation of the different materials, as shown in Fig. 34. 1R FRQGXFWLQJ FRQQHFWLRQ 1R VLJQLILFDQW SRWHQWLDO GLIIHUHQFH EHWZHHQ HOHFWURGHV PHWDOV (OHFWURO\WH (OHFWURO\WH 0HWDO 0HWDO 0HWDO 0HWDO ,QVXODWLRQ 1R FRQQHFWLYH HOHFWURO\WH
As a general rule of thumb, a fastener should always be made of the same or a more noble metal than the part to be fastened, since it typically has the smaller surface area, and failure of the fastener is usually critical. Table 2 shows the impact of galvanic corrosion under atmospheric outdoor conditions for various material combinations.
4 HOW TO ASSESS CORROSION IN A SPECIFIC ENVIRONMENT AND APPLICATION The following section describes ways to help determine the corrosivity of certain environments. The parameters influencing the corrosion can only be checked by specialists working locally on a specific project. Accordingly, the final decision regarding the chosen material and products is the responsibility of the user and/ or specifier.
The important variables for atmospheric corrosion are: Temperature General influence An increase in temperature leads to an increase in the rate of chemical reaction and therefore also an increase in the corrosion rate. This is especially true at constant relative humidity levels. Additional information On the other hand, increasing temperature facilitates the drying of wet surfaces and can slow down corrosion rates. At temperatures below freezing point corrosion is negligible.
Sulfur dioxide General influence Of all the atmospheric contaminants originating from industrial processes such as fuel combustion and metal smelting, sulfur dioxide is the most important one in terms of concentration and its effect on corrosion rates. Sulfur dioxide gas in the atmosphere acidifies the electrolyte on the surface and leads to the formation of soluble corrosion products. Corrosion rates are thus increased on many metals, e.g. zinc, steel, aluminum and stainless steel.
The outcome of this approach is an estimated rate of corrosion of zinc or steel in a given environment. The resulting corrosion rates define the prevalent corrosivity category (C-class, see table 3). As stated in the standard, the possible deviation using environmental data and the dose-response function may be up to 50 %. It must be noted that the results are only valid for macroclimatic and fully exposed (unsheltered) conditions.
Zn-alloy coatings like ZM have significantly reduced corrosion rates in atmospheric applications. For lifetime estimation of such coatings the DIN 55634 provides a useful selection table for general guidance. It is based on the technically sound assumption that ZM coatings have half the corrosion rate compared to pure Zn coatings.
Summing up all of the factors results in a number of points (the CRF) which then relate to five corrosion resistance classes (see Table 4). This is only an excerpt of the procedure. For details please refer to the standards. Corrosion resistance class CRC (link to the corrosion resistance factor CRF) I CRF = 1 II 0 ≥ CRF > -7 III -7 ≥ CRF > -15 IV -15 ≥ CRF ≥ -20 V CRF < -20 1.4003 1.4301 1.4401 1.4439 1.4565 1.4016 1.4307 1.4404 1.4462 1.4529 1.4512 1.4311 1.4435 1.4539 1.4547 1.
5 HOW TO SELECT A SUITABLE FASTENER AND INSTALLATION SYSTEM Information about the selection tables Hilti offers fasteners and installation systems in a wide range of suitable, cost- efficient materials.
Corrosion resistance class Stainless steel Corrosion categories according to ISO 9223:2012 In Table 6 the environmental conditions given in the tables are described in more detail. The underlying predominant corrosion categories and corrosion resistance classes are also listed.
Selection tables for fasteners and installation systems Fasteners such as anchors, nails and screws are often used for singlepoint and safety-relevant fixtures. Poor or limited visibility of the fasteners after installation, the inability to repair or replace them, the part to be fastened, and (in the case of expansion anchors) the friction conditions in the drilled hole all mean it is necessary to take a conservative approach to minimize the risk of corrosion to the fastener material.
5.1 Selecting the right corrosion protection for anchors, power-driven fasteners and screws For fasteners to be perfectly satisfactory and reliable for their entire service life from a corrosion perspective, all the influencing factors must be identified before a suitable fastener can be selected. The following table provides a general guideline for the most common applications for fastening elements.
5.
6 BIBLIOGRAPHY Internet resources General corrosion http://corrosion-doctors.org/ Zinc coatings http://www.feuerverzinken.com/ http://www.galvanizeit.org/ http://www.nordicgalvanizers.com/ Stainless steel http://www.edelstahl-rostfrei.de/ http://www.bssa.org.uk/ http://www.worldstainless.org/ http://www.euro-inox.
7 DISCLAIMER All information given in this handbook is based on the tests, principles, formulas, standards and approvals described in this handbook, current as of the date of publishing (June 2021). It applies only to applications comparable to the conditions described. Extrapolation of the results to other environments is not permissible.
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