The attractive and hygienic surface appearance of stainless steel products cannot be regarded as completely maintenance-free. All grades and finishes of stainless steel may in fact stain, discolor or attain an adhering layer of grime in normal service. To achieve maximum corrosion resistance the surface of the stainless steel must be kept clean. Provided the grade, condition, and surface finish were correctly selected for the particular service environment, fabrication and installation procedures were correct and that cleaning schedules are carried out regularly, good performance and long life will be achieved. The frequency and cost of cleaning of stainless steel are lower than for many other materials and this will often out-weigh higher acquisition costs.

Why Maintenance is Necessary

Surface contamination and the formation of deposits are critical factors that may lead to drastically reduced life. These contaminants may be minute particles of iron or rust from other non-stainless steels used in nearby construction and not subsequently removed. Industrial, commercial and even domestic and naturally occurring atmospheric conditions can result in deposits that can be quite corrosive. An example is salt deposits from marine conditions.

Working environments can also create more aggressive conditions, such as the warm, high humidity atmosphere above indoor swimming pools. These environments can increase the speed of corrosion and therefore require more frequent maintenance. Modern processes use many cleaners, sterilizers, and bleaches for hygienic purposes. All these proprietary solutions, when used in accordance with their makers' instructions are safe, but if used incorrectly (e.g. warm or concentrated) can cause discoloration and corrosion on the surface of stainless steels. Strong acid solutions (e.g. hydrochloric acid or "spirits of salts") are sometimes used to clean masonry and tiling of buildings but they should never be permitted to come into contact with metals, including stainless steel. If this should happen the acid solution must be removed immediately by copious water flushing.

Maintenance During Installation

Cleaning of new fabrications should present no special problems, although more attention may be required if the installation period has been prolonged. Where surface contamination is suspected, immediate attention to cleaning will promote a trouble-free service life. Food handling, pharmaceutical, and aerospace applications may require extremely high levels of cleanliness.

On-Going Maintenance

Advice is often sought concerning the frequency of cleaning of products made of stainless steel, and the answer is quite simply "clean the metal when it is dirty in order to restore its original appearance". This may vary from once to four times a year for external applications or it maybe once a day for an item in hygienic or aggressive situations. In many applications, the cleaning frequency is after each use.

Good Housekeeping During Manufacturing

Stainless steel can be contaminated by pick-up of carbon steel ("free iron") and this is likely to lead to rapid localized corrosion. The ideal is to have workshops and machinery dedicated to only stainless steelwork, but in a workshop also processing other steels avoid pick-up from:

• Tooling used with other metals
• Grinding wheels, wire brushes, linishing belts
• Steel storage racks
• Contamination by grinding or welding sparks
• Handling Equipment
•  Adjacent carbon steel fabrication

Cleaning Methods

Stainless steel is easy to clean. Washing with soap or a mild detergent and warm water followed by a clean water rinse is usually quite adequate for domestic and architectural equipment. An enhanced appearance will be achieved if the cleaned surface is finally wiped dry. Specific methods of cleaning areas in Table 1.

Sections below give passivation treatments for removal of free iron and other contamination resulting from the handling, fabrication, or exposure to contaminated atmospheres, and pickling treatments for removal of high-temperature scale from heat treatment or welding operations.

Passivation Treatments

• Grades with at least 16% chromium (except free machining grade such as 303), 20-50% nitric acid, at room temperature to 40^oC for 30-60 minutes.
• Grades with less than 16% chromium (except free machining grades such as 416), 20-50% nitric acid, at room temperature to 40^oC for 60 minutes.
• Free machining grades such as 303, 416, and 430F, 20-50% nitric acid + 2-6% sodium dichromate, at room temperature to 50^oC for 25-40 minutes.

Pickling Treatments

• All stainless steels (except free machining grades), 8-11% sulphuric acid, at 65 to 80^oC for 5-45 minutes.
• Grades with at least 16% chromium (except free machining grades), 15-25% nitric acid + 1-8% hydrofluoric acid, at 20 to 60^oC for 5-30 minutes.
• Free machining grades and grades with less than 16% chromium such as 303, 410, and 416, 10-15% nitric acid + 0.5-1.5% hydrofluoric acid, at 20 to 60^oC for 5-30 minutes.

"Pickling Paste" is a commercial product of hydrofluoric and nitric acids in a thickener - this is useful for pickling welds and spot contamination, even on vertical and overhanging surfaces.

Table1. Methods of Cleaning Stainless Steel

Precautions

Acids

Acids should only be handled using gloves and safety glasses. Care must be taken that acids are not spilled over adjacent areas. All residues must be flushed to a treated waste stream. Always dilute by adding acid to water, not water to acid. Use acid-resistant containers, such as glass or plastics. If no dulling of the surface can be tolerated a trial treatment should be carried out; especially for pickling operations. All treatments must be followed by thorough rinsing.

Solvents

Solvents should not be used in confined spaces. Smoking must be avoided when using solvents.

Chlorides

Chlorides are present in many cleaning agents. If a cleaner containing chlorides, bleaches, or hypochlorites is used it must be afterward promptly and thoroughly cleaned off.