How it works
On a uniformly corroding surface, anodic and cathodic sites are microscopic and shift constantly, so no single spot is attacked for long. The net effect is an even loss of thickness across the whole wetted or exposed area. Typical examples are bare carbon steel in the atmosphere or in water, steel in acids it is not resistant to, and the slow thinning of tank floors and pipe walls in mildly corrosive service. Because the attack is spread out, it is usually visible long before it becomes dangerous, unlike pitting or cracking, which can perforate or fracture a component with little overall metal loss.
Why designers like it
Predictability is what makes uniform corrosion manageable. A known rate, expressed as thickness lost per year, multiplied by the intended service life gives the extra wall thickness to add above the strength requirement: the corrosion allowance. Where the expected rate is too high for an economical allowance, the answer is a more resistant material, a coating or lining, inhibitors or cathodic protection. The weakness of this logic is that real equipment rarely corrodes perfectly evenly, and an allowance sized for uniform loss gives no protection against localized attack.
How it is measured
| Method | What it gives |
|---|---|
| Ultrasonic thickness readings | Remaining wall and corrosion rate at fixed points over time |
| Weight-loss coupons | Average rate from mass lost over a known exposure |
| Laboratory immersion tests | Comparative rates for candidate materials in a given fluid |
In service, thickness is tracked at condition monitoring locations so successive readings compare the same spot. Coupons are exposed in the process stream or the environment, then cleaned and weighed. ASTM G1 covers preparing, cleaning and evaluating corrosion test specimens, including the conversion of mass loss to a corrosion rate from exposed area, exposure time and alloy density. ASTM G31 is the guide for laboratory immersion corrosion testing of metals. Both give average rates: a coupon that also shows pits must be examined separately, because its average figure will understate the local damage.