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Salt Corrosion

October 1, 2014

As my name might indicate, I have a special attraction to Italian food. My addiction in that regard was fueled first by my mother, who, despite being of Polish ancestry, is an excellent Italian cook. Since then, I've been happily married to another excellent cook. As they say, the way to a man's heart is through his stomach.

For that reason I noticed a recent article about the Olive Garden, a chain restaurant serving Italian-American food.[1] According to a lengthy criticism of the restaurant management by a hedge fund, the restaurant is not salting the cooking water for pasta, since unsalted water allows it to get an extended warranty on its pots.[1] Every cook knows that pasta water is salted for better taste.
Spaghetti Bolognese‎Spaghetti Bolognese

Bolognese sauce is a meat sauce that's commonly served with lasagna.

There's a Wikibooks recipe for Bolognese sauce available online.

(Via Wikimedia Commons.)
From a materials perspective, it's easy to see why such a warranty would be offered. Salt, especially in a high temperature solution, is corrosive, so much so that a salt spray test is used to access the corrosion resistance of materials. According to ISO 9227, "Corrosion tests in artificial atmospheres - Salt spray tests," the material is sprayed with a sodium chloride (NaCl) solution of 50 grams per liter at 25°C.[2]

The gas turbine engines of airliners are subject to a particular type of salt corrosion known as high-temperature hot corrosion (HTHC, also known as Type I corrosion) that's caused by the presence of salts such as sodium chloride and sodium sulfate (Na2SO4) in the air, especially in marine environments.[3]

At the operating temperature of these engines, the salts are molten (sodium chloride has a melting point of 801°C), and they attack the protective oxide layers of the superalloy turbine blades.[3] Although sea air is a common source of such salt, it can also come from industrial pollution and volcanic eruptions, so now you know why the Iceland volcanoes disrupt air travel.

Architectural stonework, as well as metals, are subject to salt corrosion. A team of scientists from the Eidgenössische Technische Hochschule Zürich (Zürich, Switzerland), Sika Technology AG (ETHZ, Zürich, Switzerland), and Princeton University (Princeton, New Jersey) has examined the process by which porous building materials are damaged by the pressure exerted by salt crystals growing in their pores.[4-6]

As anyone who had had a favorite flowerpot or other container crack when retained water solidifies into ice knows, solidification can exert pressure. Older buildings, some of historical significance, often age because of the action of salt. Salts, crystallizing inside the pores of building materials, can generate enough force to break the stone. This effect occurs not just in natural stone, but also in the concrete of modern structures. The research team is doing experiments to test the action of salts under controlled conditions in an attempt to predict such weathering.[6]

Building salt corrosion, HavannaAn older building showing signs of salt corrosion in Havana, Cuba.

(Photo by Julio Llopiz/ETH Zurich.)[6]

As Francesco Caruso, a post-doctoral researcher at ETHZ explained in a press release, cement, a component of concrete, always contains the sulfate salt, calcium sulfate, commonly called gypsum, and other and alkali sulfates. Environmental salt can enter the materials by capillary action from mineralized groundwater. Sulfur dioxide, a common air pollutant, will react with the calcium carbonate in limestone to form gypsum.[6] Furthermore, says Caruso,
"If [de-icing salt and seawater spray] are dissolved by rain, the saline liquid can enter the building material through pores and cracks."[6]

As the liquid evaporates, such salts crystallize to cause the stonework to crumble.[6]

In the laboratory simulation of the process, the research team placed two-centimeter cubes of limestone in sodium sulfate solution. After the salt solution permeated the limestone pores, they dried the specimens at high temperature, then repeated the process again and again. The salt accumulated in the pores as anhydrous sodium sulfate, and the repeated cycling allowed the formation of a supersaturated salt solution in the poses from which crystallites will grow.[4,6]

Building salt corrosion, HavannaSalt corrosion damage to a building in the older section of Havana, Cuba.

(Photo by Julio Llopiz/ETH Zurich.)[6]

Not unexpectedly, the higher the supersaturation, the greater the corrosive effect. The lowest point of the temperature cycle was also a big factor. If the temperature never went below 25°C, about four cycles were required for damage to occur. However, when the temperature dropped to 3°C, damage was apparent after a single cycle.[6]

Such research findings are important, also, in the preservation of frescos, such as Michelangelo's frescos in the Sistine Chapel.[6] The effect has been noted, also, in geology. In 2007, the ground of part of Staufen im Breisgau, Germany, rose by about a foot, since geothermal drilling had allowed groundwater to entered a deposit of anhydrous calcium sulfate to form gypsum. This generated enough pressure to lift the ground.[6]

References:

  1. Jordan Weissmann, "Olive Garden Has Been Committing a Culinary Crime Against Humanity," Slate, September 12, 2014.
  2. International Organization for Standardization, ISO 9227:2012, Corrosion tests in artificial atmospheres -- Salt spray tests.
  3. N. Eliaz, G. Shemesh, and R.M. Latanision, "Hot corrosion in gas turbine components," Engineering Failure Analysis, vol. 9, no. 1 (February 2002), pp. 31-43. A PDF file is available here.
  4. Robert J. Flatt, Francesco Caruso, Asel Maria Aguilar Sanchez, and George W. Scherer, "Chemo-mechanics of salt damage in stone," Nature Communications, vol. 5, article no. 4823 (Online publication, September 11, 2014), doi: 10.1038/ncomms5823.
  5. Supplementary information for ref. 4.
  6. Fabio Bergamin, "How salt causes buildings to crumble," Eidgenössische Technische Hochschule Zürich Press Release, September 11, 2014.

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