Scale Coupons: corrosion coupons are small pieces of metal that are exposed to a corrosive environment in order to measure the rate of corrosion. They are typically made of the same material as the equipment that is being monitored, and they are placed in the same environment. The coupons are weighed before and after exposure, and the difference in weight is used to calculate the corrosion rate.
Corrosion coupons are a simple and inexpensive way to monitor corrosion. They are often used in the oil and gas industry, as well as in the chemical and power industries.
Types of corrosion coupon
- Bio Coupons
- Scale coupons
- Weight loss Coupon
Bacterial activity is a common source of corrosion problems. A wide range of bacteria such as Sulphate Reducing Bacteria (SRB), cause a variety of corrosion types and mechanisms. Monitoring of bacterial activity is required for understanding and evaluating the possibility of bacteria induced corrosion, and to control bacterial activity by e.g. biocides.
The bio coupon assembly is a simple and common tool for monitoring bacterial activity. The bio coupon assembly comprises a housing with 5 small coupons directly exposed to the internal environment of the pipe or vessel. The assembly is mounted on a bio coupon holder and retrieved at fixed intervals.
Bio coupons are used for analysis of layers caused by biological activity only. After retrieval, the coupons are placed in a specially designed airtight container, and sent to a laboratory for immediate analysis. Correct handling of the coupons after retrieval from the pipe is required in order to achieve reliable results.
Bio coupon assemblies consist of coupon housing and 5 bio coupons. A coupon holder of fixed or adjustable length is required. The complete coupon assembly is a replaceable part. Exposed surface area is approx. 0.4 cm2 (0.06sq. in.) per coupon. Coupons and coupon holders in other materials may be delivered on request.
In some oil and gas systems, solid deposits or scale must be monitored to determine the need for pigging or other maintenance activities. As scale usually forms on cavities, it is also likely to form on small sized holes. Scale coupons are similar to 3” strip coupon, but have a series of different size holes. 3” strip scale coupons are mounted in pair of two coupons on one strip coupon holder (see above).
Scale coupons are used for analysis of layers only. Scale coupons are not to be used in other than water systems. trip scale coupons should be avoided if pigging of line is required
Weight loss Coupon
A material sample of known composition, density, size and weight is exposed to an environment for a period of time after which it is recovered, cleaned and weighed. The difference in weight before and after exposure (the weight loss) is used to calculate the volume of material lost. The loss relative to the surface area gives a figure for average depth of material lost.
This result may be annualized relative to the exposure time, resulting in a rate of loss per year, usually expressed as Mils Per Year (MPY) or Millimeters Per Year (MMPY). Relevant standards are NACE RP0775 or ASTM G1. This data sheet is principally concerned with the monitoring of process systems using metallic samples but other materials may be used such as GRP.
Strip coupons are usually mounted in pairs and placed in or partially in the process flow. Disc coupons are usually mounted flush with the wall of the pipe (or vessel) so that conditions are as close as possible to those at the process/containment interface.
Scale coupons are strip shaped with a number of different sized holes so that scaling tendency can be assessed. Weld coupons are sections of a weld with HAZ and parent metal that are used to assess preferential weld corrosion. Stressed coupons have a known amount of bending stress applied by a special mounting arrangement.
They are used to assess stress cracking tendency. The most widely used configurations are Strip and Disc types. Coupons are supplied cleaned, weighed and serial numbered in sealed, corrosion inhibiting envelopes complete with a set of insulators.
The strip weld coupon is a special type of weight loss coupon, especially made for the monitoring of welds. The strip weld coupon is made from a piece of the actual weld, containing all parts of the Heat Treatment Zone. Strip weld coupons are made in 3” or 2” lengths, preferably depending on the pipe diameter of the piece the weld is taken from. The visual inspection of a weld coupon is important in addition to weight loss measurements.
Location of Coupon / Probes
The monitoring probes/coupons shall be installed in 2 inch access fittings at pre-selected locations in each pipe to be monitored within an individual distance of minimum 0.5 m (two fittings).
The probes/coupons shall be located at representative locations where there is a high probability of corrosion taking place. Unless otherwise specified, location of the probes shall be at the 6 o’clock position where water is likely to accumulate.
A retriever tool is required to install/retrieve the probes/coupons from equipment under production. Sufficient overhead space for the tool shall be allowed for in the piping layout and when selecting the sensor location.
A hydraulic retriever tool or double block and bleed shall be utilized for safety reasons when the operating pressure exceeds 5 ksi, or the H2S content is large. The sensor location shall also be easily accessible and shall not require scaffolding to access.
Based on the process data and selected material, an evaluation of process lines to be monitored shall be performed to identify candidate locations. Guidelines are given below (for carbon steel piping) with a minimum of retrievable coupons in each case:
i) All in going/out going flow lines on the platform downstream of the injection point for corrosion inhibit or Inlet of separators
ii) Water outlet from separators and desalter units
iii) Produced water lines
iv) Water injection system
v) Water systems where inhibitors are applied
vi) Closed and open hazardous drain systems.
vi) Crude oil line on FPSO from production manifold.