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Technical > Corrosion information

Stainless Steel is manditory for Wall Ties

Steel Lintels, wall ties, and masonry supports are relatively modern construction practice,

Wall ties were once galvanised, following the discovery that they could completely distintegrate within a few decades, stainless steel has been made manditory and wall tie replacement is now it's own industry.

 

Corrosion material loss in steels per 10 years in a coastal atmosphere

Grade 316 Stainless:
LDX 2101 Stainless:
Grade 304 Stainless:
SS370 Stainless:
Zinc Coating:
Mild Steel:		 < 0.001 mm
< 0.001 mm
< 0.001 mm
0.002 mm
0.025 mm
0.560 mm

The above information is from extended scientific trials of the materials in atmospheric conditions, most spanning 10+ years.
Local conditions can vary dramatically, so this should only be used as a guide to the relative corrosion resistance of the steels.

This type of data does highlight how inferior a zinc coating is to stainless steel, even though far better than mild steel

 

The Problem with Galvanising

“The corrosion resistance of a galvanised product is due to the fact zinc corrodes more readily than (in preference to) mild steel! Luckily the rate of material loss of zinc is slower than that of mild steel rusting, giving a lifespan advantage. However the zinc is still corroding, and at a much faster rate than any stainless steel will! Zinc corrosion is a white powder, which is psychologically less alarming than seeing rust form. Evidence of zinc corrosion can be seen on a galvanised lintel removed from a building in which it was installed for as little as one week!

The corrosion rate of zinc means even a product with an undamaged coating has a limited lifespan, the base metal rapidly corrodes once exposed - how many galvanised iron roofs in good repair and over 30 years old can you think of?

Even Thick zinc coatings such as ‘hot dipped’ have problems. The coating usually exhibits lower adhesion than a mill applied finish, and it may not penetrate narrow crevices (where water will) at all. Stainless Steels do not have such reliance on a coating for protection - the parent material is highly corrosion resistant all the way through, whereas a galvanised product is protected by a corroding coating of typically around 30-40um.

 

Zinc Coated Lintel Corrosion Example:

Many manufacturers produce lintels with a lesser zinc coating than 600gsm and then paint them to comply with the requirements of British Standards.

But a stainless steel lintel in the same application will be structurally sound for hundreds of year!

 

Lintels are installed in the same environment

Obviously steel lintels have more material, but they are similar thickness.
It is entirely possible that similar discovery of widespread lintel corrosion in the near future on older houses with galvanised lintels may lead to stainless steels becoming manditory for lintels - just as happened for wall ties.

  

Comparing corrosion performance of Stainless Steels

There are several different types of corrosion in steels, but in all cases LDX 2101 Stainless Steel outperforms grade 304 Stainless Steel. In some cases, LDX 2101 outperforms grade 316 Stainless Steel.

In our case, the most important consideration is how long the different steels will last when installed in a typical masonry building.

  

We know from decades of experience that grade 304 stainless stainless steel performs admirably in building exteriors, so we need to satisfy ourselves that the new LDX 2101 duplex performs similarly well. The various stainless steels we use have been compared using a range of standard laboratory corrosion tests:

  

Uniform Corrosion Resistance Test >>

The ISO general corrosion test involves the application of a hydrochloric acid solution. The results (together with the pitting result) give a relative indication of how the different metals are likely to perform under typical open atmospheric corrosion conditions.
They results are expressed in terms of the solution concentration required to induce a corrosion rate of 0.1mm/year.

Solution concentration required to induce a corrosion rate of 0.1mm/year:

LDX 2101 Stainless:
316 Stainless:
304 Stainless:
SS370 Stainless:
 14% H2SO4
23% H2SO4
5% H2SO4
(unavailable)
 
 

Pitting / Crevice Corrosion test >>

Pitting and crevice corrosion are similar corrosion mechanisms, resistance can be predicted using the Pitting Resistance Equivalent Number (PREN) which takes into consideration to proportions of Chromium, Molybdenum & Nitrigen in the alloy (details are in the technical pages of our catalogue). These predictions have proven approximately correct to the ASTM G 150 Avesta Cell proceedure to determine critical pitting termperature.

Pitting Resistance Equivalent Number (PREN)
%Cr + 3.3%Mo + 30%N (+16%N for Duplex)

LDX 2101 Stainless:
316 Stainless:
304 Stainless:
SS370 Stainless:
 28
26
21
12
 
 

Stress Corrosion Cracking >>

Stressed or worked areas of stainless steel can prove weak points for corrosion resistance. In short, this is tested by immersing a U-bend of steel in a CaCl solution at 100degrees C and measuring the time until stress corrosion cracks appear. The grain structure of the steel has strong influence on this result.

Hours until appearance of Stress Corrosion cracks in a 100degC CaCl Solution

LDX 2101 Stainless:
316 Stainless:
304 Stainless:
SS370 Stainless:
 >500 hours
approx. 150
approx. 150
>500 hours
 
 

Intergranular Corrosion >>

Is a corrosion condition where the boundries of the grain structure are more suseptible to corrosion and corrode first. This is due to Chromium Carbide precipitation during slow cooling. SS37 and LDX 2101 have lower carbon contents that the Austenitic stainless steels and are thus less susceptible to intergranular corrosion. This can result from the mill process or welding procedures, but in our case, all materials are relatively light gauge and thus cool rapidly, minimising carbide precipitation.

Resistance to Intergranular Corrosion

LDX 2101 Stainless:
316 Stainless:
304 Stainless:
SS370 Stainless:
 High
Moderate
Moderate
High