Carbon Low Alloy Steel Properties & Corrosion

Irons and Steels.

Irons and steels are the most commonly used materials for shore facilities. They are subject to uniform corrosion at a low rate and can be protected by protective coatings and/or cathodic protection. Due to the experience with using irons and steels in a wide variety of environments, there are large amounts of data from corrosion testing and much in-service performance data available for the irons and steels. Irons and steels are low in cost and are easy to form and fabricate.

Carbon Steels.

Steels with a total alloy content less than 3% corrode uniformly in a wide variety of environments including atmospheric and immersion service. The corrosion of carbon steels in marine environments varies widely and is dependent on local conditions. Corrosion Rates as low as 1 MPY and as high as 250 MPY have been experienced. For immersion service in seawater, the rates of uniform attack are less dependent on local conditions and rates from 5 to 10 MPY are common. Carbon steel is seldom exposed to aggressive environments without coatings or cathodic protection. It is the performance of these systems rather than the corrosion rates of the unprotected material that govern the life of carbon steel marine equipment. Temporary localized failure of protective systems does not result in high rate localized attack. Carbon steel is anodic with respect to most other materials (See Galvanic Series of Metals in Seawater or Galvanic Corrosion Chart and Galvanic Corrosion).

High Strength-Low Alloy Steels.

High strength-low alloy steels show improved corrosion resistance over carbon steels in rural and mild industrial environments. In  marine atmospheres and in immersion services, however, the difference in performance between carbon and low-alloy steels is minor (see Seawater Corrosion & Material Selection). The primary advantage of these materials is their higher strength. But remember that the same amount of material loss will usually have a greater impact on the load carrying capacity of a high strength material than on a low strength material. The high strength-low alloy steels should be protected when used in marine environments. They are somewhat more cathodic than carbon steels.

Alloy Steels.

Steels with an higher alloy content are more susceptible to pitting corrosion attack than steels with lower alloy content. Pitting is common in alloys with more than 5% total alloy content. Corrosion rates are similar to carbon and low alloy steels with pitting being only three to five times the corrosion rate calculated from weight loss.

Alloy steels are selected for their higher strength but can be susceptible to hydrogen embrittlement or stress corrosion cracking at yield strengths in excess 100 ksi. The alloy steels are somewhat more cathodic than carbon steels.

Cast Irons.

“Cast iron” refers to a wide variety of materials. Commonly, they are ferrous alloys that contain more than 2% carbon that is found in the alloy such as graphite or large carbide aggregates. These alloys are used primarily for their formability by casting rather than their mechanical properties or corrosion resistance. Many of the low alloy cast irons are subject to graphitization.

This occurs fairly uniformly with a penetration rate of 0.010 to 0.020 inches per year in seawater immersion or in many soil environments. Alloy cast irons containing 20% nickel have improved corrosion resistance and corrode uniformly at a low rate. The potential of graphitized cast iron can be very cathodic with respect to most other materials and can approach that of graphite.

Thus, when graphitized, cast irons can cause corrosion when electrically coupled to most other metals. Alloy cast irons are somewhat more cathodic than carbon steels. (See Galvanic Series of Metals in Seawater or Galvanic Corrosion Chart and Galvanic Corrosion)