The main alloy element of stainless steel called "Chromium", and the stainless steel should havecontained more than 12% of Chromium.

  
Nickel needs to interact with chromium in order to exert a stronger corrosion resistance, as chromium has excellent anti-corrosion. 

The reasons why stainless steel products that used in the home have rust spots on it:
1.The old pipelines are the key factor that caused rust, as the tap water contains high minerals.
2. It could be caused by ingredients contamination such as soy sauce, vinegar, BBQ sauce, salad dressing, etc.
3. It could be caused by chemical contamination such as kitchen detergent, floor cleaner and etc.

Stainless steel is known as "white iron" in Taiwan. The surface of stainless steel should be washed with water after it has been cleaned with strong detergent. Different stainless steel materials will have different damage. 

Whether it is grade #430 or a special steel with the highest nickel content, it will never rust. 

Grade #304 steel is more expensive than #430 as it contains more nickel and its corrosion resistance is much stronger. 

Due to no pollutants inside the bulding, stainless steel will not get rusty. On the other hand, it will get rusty if placed outside the bulding, as acidic particles or other chemical components in the air contaminate the surface of stainless steel to get rust spots, as this sort of condition is caused by external factors. 

The reason why there are slightly magnetic on corners of the kitchen sink and around the sink cage of grade #304 stainless steel?    

The kitchen sink is using a flat plate that shaped by under the pressure of 500-600 tons, or even 1200 tons.    

The steel itself has conducted slightly magnetic after the changes of it's four corners as being formed by a punching machine.          

In which is the principle of normal physics, the material itself has not been any changed.

 Stainless steel is an alloy that is combined with various metal elements, and is not a natural element. 
The most important element that determines the degree of corrode prevention of stainless steel is Chromium (Cr.) . 
According to international standards, any alloy contain chromium of more than 10.5% will be attributed to the category of stainless steel. 
As stainless steel is resistance to corrosion results from the chromium, which forms a passive layer that can protect the material by thin oxide.  
The most common type:    
(1)#304 Stainless Steel    
Grade 304 is a stainless steel that is 18-20% chromium and 8-10% nickel. Known as "white iron" in Taiwan, also, it is usually referred to as 18/8 or 18/10 stainless steel on the market.
Feature of #304：        
1.Good ductility: It can be processed into various utensils, such as tableware, pot, and sink.
2.High resistant to rusting: Excellent acid and alkali resistance.
3.Easy surface treatment: Produce for BA, 2B, hairline and etc.
4.Corrosion resistance: Have a better corrosion resistance when subjected environmental erosion.
5.Stable material: It is not easy to precipitate heavy metal elements that is harmful to human body when heated.
6.Raw material is not magnetic.

(2)#202 Stainless Steel                                    
Grade 202 is a stainless steel that is 17-19% chromium and 4-6% nickel. It often becomes a substitution for grade 304 stainless steel due to lower price.
1.Basic resistant to rusting: Basic acid and alkali resistance.                    
2.Easy surface treatment: Produce for BA, 2B, hairline and etc.                    
3.Basic corrosion resistance: Have a better corrosion resistance when subjected to environmental erosion.    
4.Raw material is not magnetic.                                
5.It contains 7.5-9% manganese element (MN), which is easy to precipitate after heating, so it is not suitable for tableware.

(3)#430 Stainless steel        
Grade 430 is a stainless steel that is 16-18% chromium and 0% nickel, known as nickel-free stainless steel.
1.Basic resistant to rusting: Basic acid and alkali resistance.
2.Surface treatment is not that easy: The surface gloss is dark, disadvantage for surface treatment.
3.Poor corrosion resistance: Poor corrosion resistance when subjected to environmental erosion.
4.Raw material is nickel-free and magnetic.

                                                              
There are about 180 types of stainless steel grades, of which the most common ones are #304 and #430 stainless steel and the recently developed 200series.                                                                        
For the standard specification of stainless steel, every national government has formulated their own market standards.                                                                         
The United States is represented by AISI, China is represented by GB, Japan is represented by JIS,  Germany is represented by DIN and Taiwan is represented by CNS.                                                                        
For example, the most common grade 304 stainless steel, Japanese regulations will use #304 to represent #304 stainless steel (S is steel and US is stainless), and American regulation is represented as AISI 304.                                                                         
While China and European countries use other marking methods. In addition, since #304 is 18% chromium and 9% nickel.                                                                        
Therefore, people refers grade 304 as 18-9 stainless steel on the market.                                                                         
The main reason why stainless steel does not get rusty is due to its high content of nickel (Ni).                                                                         
The 300 series have the highest nickel (Ni) content. which can effectively protect against external erosion.                                                                         
200 series have the second highest nickel (Ni) content with a higher percentage of manganese content.                                                                         
It is easy to harm the human body when use ultra-manganese products in the long term and may increase the risk of neurodegenerative diseases.                                                                          
For the 400 series steel, it has the lowest percentage of nickel, therefore, it is easy to get rusty than the 300 series. Obviously, the best stainless steel is the 300 series.                                                                         
However, some dishonest vendors tend to use 200 series with high manganese content in order to reduce the cost. It was advised that everyone should pay attention of this issue.