The Corrosion may happens in Humid Way or High Temperature.

CORROSION IN HUMID WAY

• General Corrosion:

It’s the corrosion that comes up in the whole surface causing uniform loss of the thickness. To increase the resistance to this type of corrosion it’s suggested to raise the Cr (Chromium), Ni (Nickel) and Mo (Molybdenum) texts, beyond the addition of Cu (Copper).

• Holes Corrosion: 
  

It’s a located attack and it occurs in recesses, in socket, holes and other spaces where a corrosive agent is accumulated. Additions of Cr, Mo and N (Nitrogen) increase the resistance to the corrosion

• Located corrosion / Puff / Alveolar:
  

It’s characterized by an attack located in a limited area, presenting an important perforation, while the neighboring regions remain intact. The most common cases of this type of corrosion occur in metallic pieces immersed in the ocean. Additions of Cr, Mo and N increase the resistance to the corrosion.

• Corrosion Under Breaching Tension (CUBT):

It’s characterized by the association of three factors: residual tensions in the material, environment having chlorides and temperatures above 60ºC. The CUBT evidences by the appearance of radial snaps that spread quickly. To combat this corrosion, it is used material having a high Ni text, like Duplex steel stainless.

• Intergranular / Inter-crystalline Corrosion: 
  

It’s caused by the Chromium carbonets precipitation in the visible signals on the surface. This type of corrosion is a great danger, because it can increase substantially without being noticed. To prevent this type of corrosion, the use of “L” leagues is indicated, since they have extra-low C texts (below 0,035%); or use stabilized materials to Ti (Titanium), Nb (Niobium) or Ta (Tantalium).

• Galvanic Corrosion:

It occurs when there is a meeting of two metals that present different electric potentials. These different-metal contacts must be prevented.

• Corrosion Erosion:

It occurs when the metal is under a corrosive environment and a process of mechanical consuming. In this in case, the passive film is continuously under a corrosive and abrasive effect simultaneously. The Duplex’s stainless steel are resistant to this type of corrosion.

The resistance of stainless steel to the corrosion in high temperatures depends of many factors like the surrounding environment, the pieces or equipments manufacturing process, the cycle of operation, etc.

• Air and Oxidizing Gases in General:

The attack by oxidizing gases is probably the most frequent cause of stainless steel corrosion in high temperatures. The attack provokes, from a certain temperature, the formation of a thick oxide crust. This one is substantially affected by the circulating gases composition. The oxidation temperatures, in continuous works and in intermittent works, mentioned in stainless steel catalogues, normally are determined in pure atmospheric air, and exempt of sulphurated gases, and these must be considered as guiding indications. It’s important to known this fact when electing steel, because the presence of contaminating substances eventually reduces considerable the oxidation temperature.

• Reducing Gases:

They affect stainless steel by many ways and each case must be studied itself.

• Sulphurated Gases:

1. Oxidizing – these gases are mostly less harmful than the reducing (above). However, its presence reduces between 100 to 200ºC, or even more, in the oxidation temperature of stainless steel exempt of nickel or having a low text of this element.
2. Reducing – these gases, like H2S, are highly corrosive, mainly to steels that contain nickel. For this reason, the austenitic stainless steels are not recommended for applications that involve the presence of reducing sulphurated gases.

 

• Melted Metals e Melted Salts:

This corrosive environment acts in the stainless steel surface, also creating an eutectics of low fusing point.

Is the stainless steel main composing element, because it is essential in the passive film formation. Other elements may improve its effectiveness on the formation and maintenance of this coating, but not replace it. The bigger the Cr rate, the bigger is the corrosion resistance.

Second most important stainless steel element. Effective to regenerate the passive film (repassivation). It stabilizes the austenite in ambient temperature, which supports the corrosion resistance, and potentializes the stainless steel workability.

Blended with Chrome, is effective on the passive film stabilization in the chlorides presence. Mo additions increase the general corrosion resistance per pit in stainless steel.

In moderate amounts, has the same effect as Nickel in the alloy, however, the change of Ni by Mn is not practical. To improve heated flexibility, Mn blends with S (Sulfur) creating manganese sulfide.

C Rates of 0,03% provide greater corrosion resistance to stainless steels. Its an element which grants thermal processing hardening to martensite steels, further on promoting mechanical resistance in high temperature applications. C is bad to corrosion resistance due to its reaction with Cr (Chrome), in case of other applications. In ferritic steels, the C rate increase causes a fall in the tenacity.

All these elements are added to stainless steel for their bigger affinity with C, which avoid precipitation and the formation of Chrome Carbide, thus increasing the intergranular corrosion resistance.

Increases the mechanical resistance and corrosion resistance per pit in austenitic steel. However, N is prejudicial to ferritic steels mechanical properties.

Can be added to stainless steels, aiming to improve its factoring.

This element potentializes the general corrosion resistance in environments that contain phosphoric or sulphuric acid.

Aluminium increases the oxidation resistance at high temperatures.