The corrosion of stainless steel is not immune to it, but it is resistant to it. The conditions that encourage corrosion in stainless steel pipes were discussed previously. Stainless steel corrosion often manifests as localised corrosion rather than uniform corrosion (or “general” corrosion). Stainless steel is particularly vulnerable to pitting and stress corrosion cracking. As a result, plant managers should pay attention to corrosion in stainless steel piping systems.

In this article, we examine some common methods of detecting corrosion in stainless steel pipes and tubes. There are a variety of non-destructive testing (NDT) methods available for detecting corrosion, each with its own advantages and disadvantages. 

A cost-effective and practical method for detecting corrosion in stainless steel is incredibly challenging. The most appropriate corrosion detection method(s) may require the assistance of corrosion detection experts. The majority of stainless steel in use is austenitic stainless steel (or 300 series), which is susceptible to corrosion.

Eddy current NDE is the most effective method for detecting hidden corrosion in electrically conducting materials. A probe coil generates a localised alternating current field in the sample. The response of a material to induced eddy currents is measured by a detector or probe coil.

Stainless Steel Detection Considerations

To shed some light on the underlying principles of corrosion detection in stainless steel tubing, we first identify three differences between corrosion detection in stainless steel tubing and corrosion detection in other materials.

1. Stainless Steel Magnetic Properties

Most stainless steels (but not all) do not possess the magnetic properties of ferromagnetic materials, such as carbon steel and many other iron alloys. The majority of stainless steel produced is austenitic (or 300 series) and not ferromagnetic. In the case of non-ferromagnetic materials, corrosion detection methods relying on magnetic interaction with the piping are inapplicable. Material that is non-ferromagnetic, such as austenitic stainless steel, cannot be tested with magnetic particle testing, such as Magnetic Flux Leakage Technique (MFL).

According to experts 1, stainless steel can be detected using the following methods: Acoustic Emission, conventional Eddy Current Testing, and Infrared Thermography. Optical testing, penetrant dye testing, radiographic testing (X-ray), visual inspection, and ultrasonic testing.

2. Detecting Pitting and Cracking

Furthermore, many corrosion detection methods fail to detect pitting and stress corrosion cracking. These are both difficult problems to detect and could lead to catastrophic failures. The collapse of the Silver Bridge was caused by stress corrosion cracking, for example. It is important to use appropriate detection methods when there is a high likelihood of failure or pitting or cracking. According to experts 1, stainless steel cracking and pitting can be detected using penetrant dye testing, acoustic emission testing, ultrasonic testing, and eddy current testing.

3. Insulation Removal

In addition, many corrosion detection methods require direct contact with the metal. It is therefore necessary to remove some insulation if there is any. When insulation pieces are removed, moisture can get underneath, increasing corrosion risk. Insulation plugs that are not properly replaced are a major contributor to energy inefficiency in piping systems.

Therefore, there are few situations in which corrosion under insulation (CUI) can be accurately and thoroughly checked without cutting into the insulation.

Popular Detection Methods

We will briefly review some of the most popular methods for detecting corrosion in stainless steel after identifying unique aspects.

Visual Inspection

Corrosion can be detected by looking at it with your own eyes. Additionally, it may be the most cost-effective approach if you only have a few tubes or pipes. In large systems housing stainless steel tubes and pipes, visual inspection becomes less cost-effective due to the enormous amount of labour required. Additionally, if there is insulation, you cannot visually inspect anything you don’t cut off, making it extremely difficult to detect non-uniform corrosion. In addition, the human eye is notoriously inefficient at detecting stress corrosion cracks, which can appear incredibly small at first. It is almost never recommended to rely solely on visual inspection.

X-Ray

Corrosion can be detected with X-rays (radiography) without removing insulation. X-ray tests are impractical in many environments due to radiation from X-rays. In addition, X-rays cannot detect cracks and pits, but they can detect other kinds of defects very well.

Eddy Current Technique

Recent years have seen the Eddy Current Technique (ECT) become the most frequently recommended corrosion detection method for stainless steel. By using electromagnetic induction, alternating “Eddy” currents are applied to the pipe or tube. During testing, the coil in the testing probe changes impedance when the electromagnetic field interacts with the material.

On stainless steel, Conventional Eddy Current is the most commonly used Eddy Current method. Eddy currents are versatile, fast, and cost-effective. Conventional ECT requires removing at least some insulation from the pipe or tube in order to reach the material being tested.

Pulsed Eddy Current testing has the advantage of not requiring direct contact with the material under test. Therefore, no insulation needs to be removed. As a result, pulsed Eddy Current has become a popular method for detecting corrosion under insulation. Even so, it is not considered reliable for detecting ultra-localized corrosion, such as pitting.

What is Corrosion Inspection

A corrosion inspection involves evaluating and monitoring equipment components, structures, process units, and facilities for corrosion signs. The purpose of monitoring programs is to detect certain conditions in order to extend the useful life and serviceability of assets, as well as to improve safety and reduce replacement costs. Materials and corrosion types are all monitored in corrosion monitoring.

Insulation inspection ports 

Tanks and vessels are equipped with insulation inspection ports. Banks Industrial Group offers insulation inspection ports for inspecting pipes and storage tanks for corrosion under insulation. AKA inspection plugs, insulation inspection ports provide easy access to a pipe/vessel surface for periodic inspections

Inspection Method

A variety of inspection methods are available, including visual inspection, microscopy, liquid or dye penetrant inspection, magnetic particle inspection, eddy current testing, x-ray or radiographic inspection, and ultrasonic inspection.

Inspection plugs insulation

An insulation inspection port kit, also known as an inspection plug, allows for easy access to a pipe or vessel surface for periodic inspection and testing. Insulated pipes, vessels, tanks, and process equipment can have inspection plugs installed on the outside of their insulation jackets. Upon installation, inspection plugs provide easy access to equipment without having to remove jacketing and insulation. There are insulation inspection port kits available for various insulation thicknesses, metal jacketing types, and service temperatures.

How to detect insulation 

Test it by touching it. A warm, dry feeling should be felt in your home’s interior walls, ceilings, and floors. Lack of insulation causes damp or cold drywall and paneling inside a home. A home’s exterior wall should feel cold when touched because insulation keeps warm air inside.

Conclusion:

Corrosion detection methods are just the tip of the iceberg. For detecting CUI in stainless steel piping, none of the techniques appears to be a clear winner. Inspection needs can, however, be met more effectively by combining techniques.

Corrosion and detection methods should be considered before installing piping and insulation, especially in stainless steel pipes, as CUI can cause serious problems. Prepare a detection plan before installing the system, considering how corrosion-prone your conditions are and how costly a failure would be. Consider corrosion detection and prevention when choosing your insulation system. 

It is best to use removable insulation when frequent inspections and maintenance are needed. Corrosion detection experts should also guide the choice of detection methods and the planning of corrosion detection.