1 STAINLESS STEEL IN CONSTRUCTION Cátia Maria Abbott Ferreira de Sousa Instituto Superior Técnico, Lisbon Abstract The main purpose of this paper is to analyze the concept of stainless steel and how it can be useful in construction, particularly in structural elements. To achieve this, its constitution is analyzed throughout this article, the principal alloying elements, properties and behaviors specified in European Norms (EN) and the North American Standards (ASTM), and evaluate how this material, in certain situations, can offer more advantages than carbon steel commonly used in reinforced concrete structures. This article will also discuss two experimental trials that were done on two types of austenitic stainless steel. The first test analyzes the mechanical characterization by tensile tests and the second test evaluates the susceptibility that stainless steel has corrosion relative to carbon steel. Key-Words: Stainless steel; Alloying element; Normative references; Structure durability; Corrosion prevention; Mechanical properties; Economical availability. 1. Introduction The oxidation of the steel caused by the phenomenon of corrosion leads to an increase volume of reinforcement that causes delamination of the concrete in the coating zone damaging in this way the reinforcement. In general, corrosion is caused by the physical-chemical interaction of a metal with its environment that results in significant changes in the properties of metal and often degradation thereof. This is therefore an electrochemical process usually spontaneous, which ally or not the mechanical stress affects the durability and performance of materials (Louro, 2008). There are numerous techniques for preventing corrosion of reinforcement in order to address the mechanism of deterioration of the concrete such as, for example, the correct choice of the thickness of the concrete coating. When the lifetime of the structures is very high, there are several methods that can be applied directly to the concrete ensuring the durability thereof, such as, for example, reducing the water/cement ratio of the concrete. However, to use these solutions it necessarily implies paying for the high cost of maintenance and repair, as these solutions do not entirely eliminate the occurrence of corrosion. The replacement of carbon by reinforcement bars of stainless steel means greater longevity of the structures and minimizes the impacts associated with the monitoring and maintenance of concrete structures. Furthermore, although stainless steel appears as an alternative solution to methods of prevention, they still have certain limitations, primarily related to economic difficulties. According to (Tula and Helene, 2000), the stainless steel reinforcement is 4 to 13 times more expensive than carbon steel reinforcement, which directly affects the initial costs of the works. Stainless steel is a very versatile material, with excellent mechanical characteristics associated with a high corrosion resistance. This is a 100% recyclable material, thus enabling sustainable development and the preservation of the environment. The durability of stainless steels is based on the principle that they are alloys of iron and chromium or iron, chromium, nickel, and other elements, which contain enough quantity to
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STAINLESS STEEL IN CONSTRUCTION
Cátia Maria Abbott Ferreira de Sousa
Instituto Superior Técnico, Lisbon
Abstract
The main purpose of this paper is to analyze the concept of stainless steel and how it can be
useful in construction, particularly in structural elements. To achieve this, its constitution is
analyzed throughout this article, the principal alloying elements, properties and behaviors
specified in European Norms (EN) and the North American Standards (ASTM), and evaluate
how this material, in certain situations, can offer more advantages than carbon steel commonly
used in reinforced concrete structures. This article will also discuss two experimental trials that
were done on two types of austenitic stainless steel. The first test analyzes the mechanical
characterization by tensile tests and the second test evaluates the susceptibility that stainless
The oxidation of the steel caused by the phenomenon of corrosion leads to an increase volume
of reinforcement that causes delamination of the concrete in the coating zone damaging in this
way the reinforcement. In general, corrosion is caused by the physical-chemical interaction of a
metal with its environment that results in significant changes in the properties of metal and often
degradation thereof. This is therefore an electrochemical process usually spontaneous, which
ally or not the mechanical stress affects the durability and performance of materials (Louro,
2008).
There are numerous techniques for preventing corrosion of reinforcement in order to address
the mechanism of deterioration of the concrete such as, for example, the correct choice of the
thickness of the concrete coating. When the lifetime of the structures is very high, there are
several methods that can be applied directly to the concrete ensuring the durability thereof,
such as, for example, reducing the water/cement ratio of the concrete.
However, to use these solutions it necessarily implies paying for the high cost of maintenance
and repair, as these solutions do not entirely eliminate the occurrence of corrosion.
The replacement of carbon by reinforcement bars of stainless steel means greater longevity of
the structures and minimizes the impacts associated with the monitoring and maintenance of
concrete structures. Furthermore, although stainless steel appears as an alternative solution to
methods of prevention, they still have certain limitations, primarily related to economic
difficulties. According to (Tula and Helene, 2000), the stainless steel reinforcement is 4 to 13
times more expensive than carbon steel reinforcement, which directly affects the initial costs of
the works.
Stainless steel is a very versatile material, with excellent mechanical characteristics associated
with a high corrosion resistance. This is a 100% recyclable material, thus enabling sustainable
development and the preservation of the environment.
The durability of stainless steels is based on the principle that they are alloys of iron and
chromium or iron, chromium, nickel, and other elements, which contain enough quantity to
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ensure the presence of the passivation mechanism, this being responsible for the high corrosion
resistance of these steels, regardless of the pH of the concrete.
The term "steel" does not designate a particular material, but a remarkably diverse set of
materials, where hundreds of different alloys, which are different in terms of chemical
composition and crystalline microstructure properties and areas of application (Colaço, 2005).
Depending on the chemical composition and the temperature to which it is subjected during the
manufacturing process, the steel may have different microstructures, which correspond to
different mechanical properties.
The alloying element iron, from room temperature up to 912ºC, the iron atoms are arranged in a
cubic body-centered microstructure, which is designated as ferrite or phase-α. Since 912ºC up
1394ºC, iron undergoes a series of transformations and its crystalline microstructure changes to
face-centered cubic structure, also called austenite or phase-ɣ. From 1394ºC up to melting
point, the iron passes again having body-centered cubic microstructure (Colaço, 2005).
Given that the carbon is considerably smaller than the iron atom, the carbon atoms can occupy
the interstices formed by iron atoms, in this way giving greater yield strength and tensile
strength of the material (Colaço, 2005).
With respect to alloying elements, when used together these may lead to slightly different
properties of steel than if they had been used separately. However, the chromium is the most
important alloying element in stainless steel.
For a steel can be classified as stainless steel, this has to have in their chemical composition
minimum chromium content of 10.5% and maximum carbon content of 1.2%, in order to create
the necessary conditions for the formation of the protective chromium oxide layer which gives
this class of steels greater corrosion resistance (IMOA, 2009).
The alloying elements can be divided between gamagenes and alphagenes, depending on the
swelling of the austenitic or ferritic field area, respectively (Colaco, 2005). Gamagenes alloy
elements, such as nickel, manganese and nitrogen have the function of stabilizing the austenite
of the steels thus enabling the microstructure centered cubic crystal faces. They are elements
that provide good corrosion resistance and good mechanical properties. Alfagéneos alloy
elements, such as silicon, aluminum, molybdenum and tungsten, whose function is to decrease
the hardenability of the steel and gives them a better mechanical strength when worked at high
temperatures. In the case of molybdenum and tungsten, these also have the function of
improving the flow of the steels and improve corrosion resistance by stabilizing the protective
passive layer of the stainless steels, especially in rich environment of chloride
The standard EN 10088-1: 1995 "Stainless Steels - Part 1: List of stainless steels" [n1] asserts
that stainless steels can be grouped into five main categories, according to their microstructure:
ferritic steels, martensitic steels, percipitation steel-hardening, austenitic steels and austenitic-
ferritic steel which generally designate as duplex.
The ferritic stainless steels, they are characterized by having very low contents of carbon and
chromium contents always higher than 16%, where the steels end up with a degree of
resistance relatively low. The corrosion resistance of this class of steels is higher than that of
martensitic but lower than that of austenitic and duplex steels. The martensitic stainless steels
contain carbon levels a little higher with respect to ferritic steels, reflecting into higher strength.
The mechanical strength of martensitic steel can be increased by heat treatment, giving rise to
a class of hardened stainless steels by precipitation. The austenitic and duplex stainless steels
have good corrosion resistance and high strength, which makes this type of steel in a good
choice for the building industry, namely in buildings structural part [n1].
When we talk about stainless steels it is very important that you understand the definition of the
phenomenon of corrosion. In the case of metals, corrosion is their oxidation and this is an
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electrochemical phenomenon. However, corrosion can happen in many ways, in which the
correct identification of the type of corrosion is essential to assess the mechanism and the
correct application of protective measures or treatment. However, there are some corrosive
processes that are beneficial and of great importance in the metal industry, such as the case of
oxidation of stainless steel which is responsible for the formation of the protective chromium
oxide film process (Gentil, 1996). Stainless steel can suffer six distinct processes of corrosion:
uniform corrosion, pitting, crevice corrosion, intergranular corrosion, stress corrosion and
galvanic corrosion.
2. Standardization and application
2.1. European norms (EN)
All European Standards relating to stainless steel supports, in general, the physical, chemical
and mechanical properties, allowable dimensional tolerances, the information that the buyer
must provide, tests, certifications, a precise description of all products and standards quality.
The aim of the European Standards is to make the market of stainless steel throughout Europe
easier and more intuitive, to minimize errors during the ordering process.
The publication EN 10088-1: 1995 "Stainless Steels - Part 1: List of stainless steels" [n1], lists
and describes the various existing classes of stainless steel, providing its chemical composition,
various physical and mechanical properties. The publication EN 10088-2: 1995 "Stainless
Steels - Part 2: Technical delivery conditions for sheet/plate and strip for general purposes" [n2],
describes the properties and terms of delivery for products in sheet form and contains 68 types
of stainless steel. This standard refers essentially to the layout application that companies
should do to the metallurgical enterprises. It also refers to the permitted tolerances. The
publication EN 10088-3: 1995 "Stainless Steels - Part 3: Technical delivery conditions for semi-
finished products, bar, rods, and section for general purposes" [n3], describes the technical
conditions of delivery of semi-finished products, plates / strips rolled sheets and hot or cold, as
well as bars, wire and rolled profiles processed hot or cold, made in stainless steel.
2.2. North American Standard (ASTM)
The ASTM A955, “Standard Specification for Deformed and Plain Stainless Steel Bars for
concrete Reinforcement” [n4] covers deformed and plain stainless steel bars for concrete
reinforcement proposed to be used in applications requiring corrosion resistance or controlled
magnetic permeability. For each specimen, one tension test, one bend test, if required, and one
set of dimensional property tests shall be made.
The publication ASTM A276 - 13, "Standard Specification for Stainless Steel Bars and Shapes"
[n5] refers to the chemical composition of stainless steels and the respective mechanical
requirements.
The ASTM A240, "Standard Specification for Chromium and Chromium-Nickel Stainless Steel
Plate, Sheet and Strip for Pressure Vessels and for General Applications" [n6] refers to
chromium, chromium-nickel and chromium-manganese-nickel in stainless steel with the form of
plates and sheets for general applications. The standard indicates that the steel must conform
to the chemical composition requirements specified in this standard. Beyond the chemical
composition requirements, the material must also comply with specified mechanical properties.
The ASTM A480, “Standard Specification for General Requirements for Flat-Rolled Stainless
Steel and Heat-Resisting Steel Plate, Sheet and Strip” [n7] covers general requirements for flat-
rolled stainless and heat-resisting steel plate, sheet, and strip.
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The ASTM A555, “Standard Specification for General Requirements for Stainless Steel Wire
and Wire Rods” [n8] covers general requirements that shall apply to stainless wire and wire
rods. The material may be furnished in one of the conditions detailed in the applicable material
specification, that is, annealed, bright annealed, or cold worked.
The ASTM A564/A564M – 10, “Standard Specification for Hot-Rolled and Cold-Finishes Age-
Hardening Stainless Steel Bars and Shapes” [n9] covers bars and shapes of age-hardening
stainless steels. Hot-finished or cold-finished rounds, squares, hexagons, bar shapes, angles,
tees, and channels are included.
The ASTM A580-13a, “Standard Specification for Stainless Steel Wire” [n10] covers stainless
steel wire, except the free-machining types. It includes round, square, octagon, hexagon, and
shape wire in coils only for the more commonly used types of stainless steels for general
corrosion resistance and high-temperature service.
The ASTM standards, such as European Standards, these are indispensable tools in the
classification, chemical and mechanical evaluation and definition of the metallurgical properties
of different types of stainless steel. These standards serve only to guide laboratories, producers
and buyers to ensure the quality and safety of the product.
2.3. Application
Architects and engineers are, increasingly, enjoying the various benefits offered by the unique
combination of properties of stainless steels for a large number of applications in construction.
Nowadays, the market of metallurgical stainless steels offers a wide range of sizes and various
kinds of resistance, which allows this to be easily used in all kinds of works.
There are currently numerous examples of bridges that were built with stainless steel elements.
An example is the Helix pedestrian bridge in Singapore.
For environments such as those prevailing in Singapore, it was essential that the chosen stainless steel had excellent long-term corrosion resistance. Several alloys could achieve this requirement but grade EN 1.4462 (S31803) provides more than adequate resistance while meeting other requirements of good availability, cost effectiveness, ease of fabrication and the structural/architectural requirements. It also provides good fatigue strength as well as high resistance to stress corrosion cracking [s1].
Currently, Stainless steel is commonly used in building roofs. An example for this type of work is
the most coverage constructed with stainless steel, which is located in the New Doha
International Airport, Qatar and came into operation in 2012. The wavy roof was built with
duplex stainless steel enriched with molybdenum (S32003) [s2].
There are many other applications in which the stainless steel has an essential role as it is the
case of units of flue gas desulfurization and desalinization plants. These are environments that
present one of the most demanding challenges for materials due to them being highly
aggressive environments. For this type of works it is often used duplex stainless steel due to its
high mechanical strength, high corrosion resistance and high toughness [s2].
Stainless steel is a material that is able to combine an extraordinary mechanical capacity at high
corrosion resistance, however, for certain environments and under certain conditions, there
must be some measures that should be taken to avoid of structures collapsing.
In 1985, a concrete cover of a pool collapsed in Uster, Switzerland. The structure had 13 years
of use. The roof was supported by pillars of stainless steel EN 1.4301 (AISI 304), which had
collapsed due to corrosion phenomena. Investigations following the tragedy came to
demonstrate that the collapse was due to the hot and humid environment rich in chlorides
associated with tension steel [s3].
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The stress corrosion cracking occurs when the steel is subjected to tension in a specific
corrosive environment. In the case of pools, the products used for disinfection of water
containing large amounts of ammonia and chlorine. These elements, when evaporated and
condensed on the steel surface, form a highly corrosive film, which can lead to stress corrosion
cracking. This type of corrosion can be controlled through the preparation of a project that
facilitates the inspection and maintenance of the structure, provided that the risk analysis
structure can provide and through more careful selection of the type of stainless steel used in
the structure [s4]. This collapse has not been the only one registered in structures of stainless
steel due to stress corrosion cracking. In the Netherlands, in 2001, a false ceiling supported by
elements of AISI 304 stainless steel in a pool fell. Two other similar accidents occurred, one in
Finland in 2002 in a hotel pool and another in the UK, also with a pool cover in 2003, where
steel was used AISI 304.
3. Experimental work
3.1. Description
To answer one of the objectives of the work, which corresponded to look to proceed for a
comparative analysis of the mechanical performance and corrosion resistance of stainless
steels and carbon steels, we proceeded to the development of an experimental campaign. The
developed experimental campaign included two parts, the mechanical characterization and
evaluation of corrosion susceptibility, carried out on steels used in the performance of
structures.
The selected materials are austenitic stainless steels of the type AISI 304 and AISI 316 The
chemical compositions percentage of both stainless steels according to EN 10088-1: 1995
"Stainless Steels - Part 1: List of stainless steels" [n1] are presented in Table 3.1.
Table 3.1 - Nominal chemical composition of austenitic stainless steel type AISI 304 and AISI 316 according to EN 10088-1: 1995 [n1].
Designation AISI/ASTM
Chemical composition [%]
C Si Mn P S N Cr Mo Ni
304 ≤0,07 ≤1,00 ≤2,00 ≤0,045 ≤0,015 ≤0,11 17,00 a 19,50 - 8,00 a 10,50
316 ≤0,07 ≤1,00 ≤2,00 ≤0,045 ≤0,015 ≤0,11 16,50 a 18,50 2,00 a 2,50 10,00 a 13,00
Carbon steels selected were the A400NR and the A500NR. The chemical compositions of the
steels A400NR and A500NR are similar, according to the specification LNEC 449-2010 [n11]
and 450-2010 [n12], Table 3.2.
Table 3.2 - Chemical composition of carbon steel A400NR and A500NR, second specifications LNEC E449-2010 [n12] and E450-2010 [n13].
Designation Chemical composition [%]
C P S N Cu
A400NR 0,240 0,055 0,055 0,014 0,850
A500NR 0,240 0,055 0,055 0,014 0,850
3.2. Mechanical Resistance
The tensile test is often used to provide basic design information on the strength of materials.
The tensile test involves subjecting a test piece to a growing and continuous axial load, while
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simultaneously registering the stretching that it is subjected to. Tensile tests were performed
according to EN 10002-1: 2006: "Metallic materials - Tensile testing" [n13].
The Table 3.3 identifies all steel and shows the number of specimens submitted to mechanical
tensile test with the objective to study their mechanical behavior. All test pieces tested had a
length (Lo) of 200 mm, Figure 3.1.
Table 3.3 - Tensile test (Part 1) - Identification of steels and the number of specimens tested.
Ø
[cm]
Steel type
Carbon steel Stainless steel
A400NR A500NR AISI 304 AISI 316
Ø 8 2 - 2 2
Ø 16 - 1 - 1
Ø 20 - 2 2 -
The Table 3.4 presents all samples analyzed of the trial and their respective identification
number, steel type, length (Lo) and the diameter (nominal and measured).
Table 3.4 - Identification of the test pieces used in the first part of the mechanical tests.
Test pieces Steel type Lo [mm] Ønominal [mm] Ømeasured [mm]
* It was not possible to measure lu due to rupture of the specimen occurred along the clutches of the INSTRON
machine.
3.3. Corrosion susceptibility
This experimental component was to study the corrosion susceptibility of austenitic stainless
steel, AISI 304 and AISI 316, relative to carbon steel, A400NR and A500NR, when exposed to a
saline solution at constant temperature and see how far the corrosion influenced or not their
mechanical performance.
After selected types of steel to make the study of corrosion and to compare the properties
chosen, proceeded to demand the necessary standards for the purpose. The corrosion test
developed corresponds to an adaptation of the procedure described in ASTM B117-11:
"Standard Practice for Operating Salt Spray (Fog) Apparatus" [n15]. Thus, there was a solution
prepared of 5% by weight sodium chloride (NaCl) and 95% distilled water, as described to this
standard.
3.3.1. Tested materials and methods
The protocol for testing susceptibility to corrosion of stainless steel involves exposing test pieces to a corrosive environment for 110 consecutive days. Then, the specimens were placed permanently to a level approximately 1 cm above the salt solution.
The test was performed in various test pieces of carbon steel and stainless steel identified in
Table 3.6, with the objective of comparing the evolution of the corrosion of stainless steels
relatively to carbon steel. All test pieces tested had a length (Lo) of 200 mm, except the two
pieces of stainless steel AISI 316 with Ø8, Figure 3.6.
Table 3.6 - Identification of steels and the number of test pieces used in the testing of susceptibility to corrosion.
Ø [cm]
Steel type
Carbon steel Stainless steel
A400NR A500NR AISI 304 AISI 316
Ø 8 4 - 5 2
Ø 12 4 - 2 -
Ø 16 - 4 - 1
The Table 3.7 shows all the samples analyzed in this test and their respective identification
number, steel type, length (Lo) and the diameter (nominal and measured).
Table 3.7 - Identification of the test pieces used in corrosion testing.
Carbon Steel
Designation Steel type Lo [mm] Weight [g] ΔM [%] Ønominal [mm] Ømeasured [mm]