The unalloyed steel from which the majority of our fasteners and support systems are manufactured requires corrosion protection. In most environments, the corrosion rate of carbon steel (typically around 20 µm/year in a rural outdoor atmosphere and rising to over 100 µm/year in coastal environments) is generally too high for outdoor applications. This material loss is generally not taken into account in the design phase. We therefore offer a wide range of steel products suitable for corrosion control, including stainless steel products.

 1- A DIFFERENT KIND OF STEEL

Stainless steel is a steel to which chromium atoms (Cr) have been added (representing at least 10% of the composition). These atoms react with oxygen in the air to form a thin invisible layer (called the passive layer) that protects the steel.

 4Cr + 3O2 → 2Cr2 O3

Even if this layer is allowed to degrade, it will reform on the surface and protect the steel. This process is called self-passivation. In some configurations, this passive layer can deteriorate when corrosion is localized (see Corrosion Basics #2, The main types of corrosion). Stainless steel (which actually oxidizes quickly, but unlike rust produces a protective oxide) does not corrode as easily as other steels. This solution being more expensive, it is advisable to use it in an adapted way. It relevant when it constitutes the only solution to resist to:

• Adverse atmospheric conditions such as carbon dioxide, humidity, electric fields, sulfur, salt, and chloride compounds
• Chemicals
• Extreme condition with very high temperatures

 1- Stainless style grades

There are several types of stainless steel that differ by the addition of various alloys. Increasing or reducing the amount of specific elements in the steel changes its corrosion properties. For example :

• Iron-chrome, carbon > 0,1% : a high carbon stainless steel makes a harder steel but degrades weldability and hinders the formation of the passive layer. This type of alloy is called martensitic stainless steel.
• Iron-chrome-nickel, carbon < 0,1% : the addition of nickel (Ni), strengthens the passive layer (i.e., is more resistant to corrosion) and provides mechanical properties superior to basic steel. This type of alloy is called austenitic stainless steel.
• A stainless steel alloyed with molybdenum (Mo) adds a barrier layer in addition to the chromium hydroxide layer, giving it high corrosion resistance (resistance to pitting corrosion improved).

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. In addition to designation 44, the following designations for stainless steel exist:

“41” = with Mo, without Nb and Ti*, Ni < 2,5 %

“43” = without Mo, Nb and Ti, Ni ≥ 2,5 %

“44” = with Mo, without Nb and Ti, Ni ≥ 2,5 %

“45” = with additional elements

*Ti : Titanium & Nb : Niobium 

The last two digits 04 designate the exact alloy.

There are other ways to designate stainless steel grade (EN steel name, AISI…). For example, Stainless steel 1.4404 can also be found as X2CrNiMo17-12-02 or 316 L.

The EN ISO 3506-1:2009 standard uses designations ranging depending of the chemical composition of the steel (from A1 to A5 for Austenitic Stainless steel) :

Stainless steel grades according to EN ISO 3506-1:2009

Therefore, two stainless steels according to EN 10088-1/2014 numbering could designated with the same steel grade when it comes to EN ISO designation. For example, 1.4401 and 1.4404 have different carbon and molybdenum rates but are both A4.

2- A SPECIFIC CORROSION RESISTANCE CLASSIFICATION

Strictly speaking, the corrosivity categories (class C) according to ISO 9223 are only applicable to zinc, carbon steel, aluminum and copper. The different corrosion mechanism of stainless steel makes it necessary to work with a different classification system than that used for zinc. In contrast to zinc, the influence of moisture without other contaminants is negligible for the corrosion of stainless steel, it is more important to consider the effect of chlorides for example. International standards and guidelines (Eurocode 3, EN 1993-1-4) generally use a special rating system to assess the suitability of certain stainless-steel grades.

This system takes into account the risks presented by the main influencing factors, from which a Corrosion Resistance Factor (or "CRF") is calculated. Each level of risk factor (chlorides, sulphur dioxide, exposure to washing) is associated with a certain number of points and then by adding the points of each factor, we obtain the CRF. There are 5 corrosion resistance classes (CRC) for stainless steels depending on their CRF level (see figure below).

List of stainless-steel grades by corrosion resistance class (CRC) based on tables in EN 1993-1-4:2006 (2014)

Designation of main stainless steels table

Furthermore, there is an "equivalence" between the corrosion categories seen above and the CRC for stainless steel:

Equivalence between corrosivity categories and corrosion resistance classes for stainless steel

An indicative classification of all anti-corrosion solutions (which takes into account the resistance to salt spray according to the NF E 25-032 standard) is available in our article Corrosion basics #3, Our anti-corrosion solutions.

3-STAINLESS STEEL AT HILTI

In our range of fasteners and support systems, you will find the most stable stainless-steel grades such as 1.4301 (in the A2 group of stainless steel), 1.4404 (in the A4 group of stainless steel) and 1.4529 (or HCR stainless steel, High Corrosion Resistance), covering all important classes of corrosion resistance:

Example of stainless-steel grades used for our products

Below is a table showing the life expectancy of our range of support systems under different environmental conditions:

 Lifetime of our support range according to the conditions

Note:
1. Life refers to the potential number of years until 5% rust appears on the surface of the most corrosion-prone component in the system (usually the threaded parts).
2. The life span is influenced by the exact environmental conditions of the project location.

To learn more about our corrosion protection options, please visit our AskHilti platform:

- Read our article on The basics of corrosion #1, What is corrosion?

- Read our article on The basics of corrosion #2, The main types of corrosion

- Read our article on The basics of corrosion #3, Our anti-corrosion solutions

You are also welcome to ask us for support: simply leave a comment or post your question in the community, or improve your knowledge and skills via our Webinars or training sessions.