Subject: Stainless Steel Is Corrosion Resistant NOT Corrosion Proof!

A few of our CBW washer customers have recently asked our advice concerning corrosion appearing at the discharge end of the tunnel and on certain parts of presses or extractors. Although our expertise is in machinery design and manufacture rather than chemicals and metallurgy, we did investigate the situation and, in conjunction with an expert in corrosion, have come up with some suggestions for preventative measures to render the system as corrosion resistant as possible.

It's important to be vigilant in maintaining a "Healthy Tunnel Washing Profile" described below. Many chemical products used improperly or misused can cause problems. Coupled with a Tunnel Washer that is operated improperly, there are arrays of problems that can manifest and compound. A Healthy Tunnel Washing Profile includes:

1. Proper supply water quality.
2. Industry standard quantity and method of injecting chemicals.
3. Proper water flow in each zone - Refer to "Quick Guide for Setting Counterflow in Milnor CBW Washers" (Attachment A).
4. Proper Levels in each module - Refer to "Quick Guide For Setting Weir Plates & Level Switches On Milnor CBW Washers" (Attachment B).
5. Proper temperature in each zone.
6. Proper pH.
7. No intermixing of sours and bleaches.

Chemical products commonly found in the laundry industry when utilized in established dosages and proper tunnel operating parameters under the auspices of an experienced tunnel chemical specialist will produce satisfactory results and no consequential detrimental effects.

It is not the steel!

Improper laundering process conditions can destroy the corrosion resistance of stainless steel. Stainless steel provides resistance to corrosion because of its capacity to be passivated. In simple terms, most scientists explain that a protective oxide film acts as a barrier between the metal and its environment (Handbook of Stainless Steels, Peckner and Bernstein, 1977).
Pitting and stress corrosion cracking can occur when the conditions of the wash liquor break down the passive layer of the steel. These conditions are well known, some of which are: high concentrations of chlorine bleach, improper (low) pH, iron-laden process water, corrosive supply water, and reactions of sours and bleach.

Milnor has manufactured washer-extractors and tunnel washers with the same stainless steel specification since our founding. Every batch of steel we use is certified and documented by the steel mill. We tested samples of the stainless after reports of corrosion. Every instance has proven the steel to be well within the AISI 304 specification.

Corrosion is a complex, multifaceted problem. When some or all of the following factors come into play together, corrosion has occurred.