New research provides an answer to why stainless steel, a staple of modern existence, can corrode, leaving a hole in a dishwasher or a catastrophic crack in an industrial plant.
The finding paves the way toward developing methods for steeling the alloy used in everything from cutlery and cooking pans to Formula 1 car engines and chemical process plants against such failure, scientists said in the British journal Nature.
Despite its resistance to rust that has led to its ubiquitous use, stainless steel -- introduced in Sheffield, England, in 1913 -- is not completely immune to corrosion, a potentially disastrous shortcoming that has been under intense investigation. One such review, by a team from University College London and Imperial College in London, has netted insight into what can go wrong and why at the molecular level.
Taking a microscopic peek at the "pitting corrosion" that has tarnished stainless steel's spit-and-polish image, the scientific inspectors observed how tiny holes form in the steel that can lead to a breakdown, particularly in the wet environments to which equipment such as dishwashers and surgical implants routinely are exposed.
Corrosion in general costs industrialized nations some 3 percent of their gross national product, scientists noted.
"Pitting corrosion is the worst kind because it's so difficult to predict or monitor," study co-author Mary Ryan of the materials department at Imperial College told United Press International.
"The problem is that the corrosion is not a general rusting, but a highly localized and apparently random attack which seems to come out of the blue and can drill through a substantial thickness of steel in a relatively short time," co-author David Williams, head of the chemistry department at University College London, told UPI. "Not surprisingly, this phenomenon has been the subject of much investigation. Now the answer has been found."
Unlike rusting, which leaves an indiscriminate reddish-brown mark on iron and steel exposed to air and moisture, stainless steel corrosion is confined to one concentrated area and can occur at random. The corrosion forms small pockets of sulfide -- left over from impurities in the ore -- which can pierce the steel, causing leaks or acting as points of origin for cracks, the researchers discovered.
"Think about scoring a piece of glass you want to break -- if you make a hole, then it's easier to crack," Ryan said. "The most dramatic failures tend to be industrial ones, typically the chemical process industry, which is one of the biggest users of stainless steel."
Stainless steel failures can also cause headaches in the home and in the hospital.
"Most of your household appliances contain stainless steel. It's quick to clean and has an attractive shiny appearance. This cleanability also makes it the material of choice for applications requiring sterile surfaces such as surgical instruments or plants for producing pharmaceuticals," Ryan said.
"Stainless steel is used in countless engineering applications and, in general, it has very good resistance and performs well, but it is susceptible to this devastating pitting corrosion," she added. "Now we've worked out the sequence of events that cause this Achilles heel, and we can use this information to work out how to fix it."
As a result, "stainless steel cladding on buildings might retain its shine better in future, and household products should become slightly cheaper to produce," Roger Newman of the University of Manchester Institute of Science and Technology in England, who wrote an accompanying News and Views article, told UPI.
The finding could help steel manufacturers modify the production process to avoid the sulfur pockets, the authors said.
"This may result in better performing, lower risk, and possible cheaper stainless steels for a given application," Ryan said.
"Heat treatment might also draw chromium back into the depleted zones" to prevent corrosion, Newman said.
Steel is endowed with its "stainless" property when iron is mixed with chromium. As the newly made steel ingot cools, minuscule sulfur-rich impurity particles solidify at a lower temperature than the steel, remaining molten after the metal turns solid.
Using instruments he had developed to study the process, Williams began to note unusual chemistry in and around the impurities, which triggered the corrosion. He theorized that the reason was the "sucking" of chromium out of the steel into the impurity during manufacture, but lacked the necessary tools to prove it.
Taking advantage of the cutting-edge Focused Ion Beam microscope -- normally used in the semiconductor industry to make integrated circuits -- the team was able to make the needed chemical measurements on the molecular scale. The investigators noted a chromium-depleted region just around the impurity particles. Just as Williams had suspected, as the steel cools, the impurity particles slurp chromium out of the steel around them, creating a "tiny nutshell" of steel lacking the stainless quality, they observed.
Corrosion of this layer, just 100 nanometers -- one-ten-millionth of a meter -- in diameter, is "the virus that triggers the main attack," Ryan and Williams noted.
"It is intriguing that something so small can be the cause of some of the most disastrous industrial corrosion failures known," Williams told UPI.
"This is a nice piece of work," Newman told UPI. "A priority is to repeat the finding."
The work is of significance to scientists and the general public, Ryan said.
"We have resolved a long-debated issue in materials science. We are using very high-resolution novel techniques to look at real engineering systems," she told UPI. "To the public, they ultimately reduce the risk of failures due to this mode of corrosion and thereby have an economic and safety impact."
